ATTftl  B  1910 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


BIOLOGY 

LIBRARY 

G 


Intracellular    Pangenesis 


INCLUDING  A  PAPER  ON 

FERTILIZATION   AND   HYBRIDIZATION 


BY 

HUGO    DE   VRIES 

PROFESSOR  OF  BOTANY  IN  THE  UNIVERSITY  OF  AMSTERDAM 


OF    THE 

UNIVERSITY 

OF 


TRANSLATED  FROM  THE  GERMAN 

* 

BY 

C.   STUART   GAGER 

PROFESSOR  OF  BOTANY  IN  THE  UNIVERSITY  OF  MISSOURI 


CHICAGO 

THE  OPEN  COURT  PUBLISHING  CO. 
1910 


C  OLOGY 

LIBRARY 

6 


GENERA 


til 


COPYRIGHT  BY 

THE  OPEN  COURT  PUBLISHING  Co. 
1910 


FOREWORD 

The  Intracellulare  Pangenesis,  of  Hugo  de  Vries,  was 
such  a  source  of  stimulation  to  me  at  the  time  of  its  ap- 
pearance that  I  feel  greatly  indebted  to  its  author.  By 
creative  imagination  Hugo  de  Vries  predicted  much  in 
his  book  that  gained  a  material  basis  only  through  the 
histological  research  of  the  following  decades.  That  is 
what  makes  the  study  of  his  book  to-day  as  interesting 
as  it  is  instructive. 

In  his  paper,  entitled  Befruchtung  und  Bastardirung, 
a  translation  of  which  is  included  in  this  volume,  de  Vries 
has  shown  the  same  faculty  of  utilizing  our  present 
knowledge  from  every  point  of  view,  and  of  looking 
prophetically  into  the  future.  For  in  this  paper  also,  on 
the  ground  of  theoretical  considerations,  he  predicted 
phenomena  which  were  to  furnish  the  basis  for  our  con- 
ceptions of  fertilization  and  heredity,  but  which  have  be- 
come actually  known  to  us  only  through  later  works  on 
the  most  intimate  processes  of  nuclear  division. 

Therefore  I  gladly  comply  with  the  wish  of  the  trans- 
lator to  introduce  his  translation  with  a  few  words.  I 
say  expressly  "to  introduce/'  for  works  of  Hugo  de 
Vries  do  not  need  a  recommendation. 

Bonn,  .  E.  STRASBURGER. 

June,  1908. 


200228 


my  well-abused  hypothesis  of  pangenesis" 

Charles  Darwin,  Autobiography. 


TRANSLATOR'S  PREFACE 

Every  student  of  heredity  is  brought  face  to  face  with 
the  problem  of  some  mechanism  of  inheritance.  Pan- 
genesis  was  Darwin's  solution  of  this  problem.  But  it 
was  not  in  the  form  in  which  Darwin  left  it  that  pan- 
genesis  became  directly  fruitful  of  results;  and  no  one 
felt  the  insufficiency  of  his  hypothesis  more  keenly  than 
Darwin  himself.  Writing  to  Asa  Gray  in  1867  he  said: 
'The  chapter  on  what  I  call  Pangenesis  will  be  called  a 

mad  dream but  at  the  bottom  of  my  own  mind 

I  think  it  contains  a  great  truth."1  And  to  J.  D.  Hooker, 
in  1868,  he  wrote :  "I  feel  sure  if  Pangenesis  is  now  still 
born  it  will,  thank  God,  at  some  future  time  reappear,  be- 
gotten by  some  other  father,  and  christened  by  some  other 
name/'2 

Many  men  discerned  the  weak  features  of  the  hypoth- 
esis, but  to  Hugo  de  Vries  belongs  the  credit  of  having 
detected  the  "great  truth"  it  contained.  He  became  its 
"other  father,"  and  rechristened  it  with  another  name 
— a  name  more  nearly  like  the  original,  no  doubt,  than 
Darwin  could  have  imagined. 

The  pangenesis  of  Darwin  was  hardly  susceptible  of 
experimental  verification,  except  to  the  extent  that  a  more 
intimate  acquaintance  with  the  facts  showed  that  the 
assumption  of  a  transportation  of  "gemmules"  was  super- 

iDarwin,  C  Life  and  Letters.  2:  256.    New  York,  1901. 
2Loc.  cit.  p.  261. 


vi  Translator's  Preface 

fluous.  But  it  contained  the  germ  of  de  Vries's  intra- 
cellular  pangenesis,  the  direct  progenitor  of  the  mutation- 
theory.  It  was  primarily  because  of  this  genetic  rela- 
tionship, together  with  the  masterful  way  in  which  the 
hypothesis  is  developed,  and  the  accompanying  wealth 
of  illustration,  that  the  little  German  volume,  here  done 
into  English,  was  deemed  worthy  of  translation  at  the 
present  time. 

As  those  who  have  followed  the  more  recent  liter- 
ature of  theoretical  biology  well  know,  Delage  has  argued 
against  accepting  any  of  the  micromeric  theories  of  the 
structure  of  protoplasm.  His  argument  is  based  upon 
the  idea  that,  by  the  law  of  probabilities,  no  one  can  ever, 
by  pure  imagination,  correctly  conceive  of  the  ultimate 
structure  of  protoplasm  in  detail.  Kellogg3  cites  this 
criticism  of  Delage  as  "a  sufficient  reason  against  accept- 
ing any  one  of  these  highly  developed  theories  of  the 
structural  and  functional  capacity  of  invisible  life  units." 
Possibly  this  is  correct,  but  that  depends  upon  what  the 
given  hypothesis  is  to  be  accepted  for.  Of  course  no 
unverified  hypothesis  should  be  accepted  for  truth.  As 
soon  as  the  hypothesis  can  be  so  accepted  it  ceases  to  be 
a  hypothesis,  or  even  a  theory,  and  passes  into  the  rank 
of  ascertained  fact. 

But  that  the  argument  of  Delage  can  be  advanced  as 
a  reason  for  rejecting  any  hypothesis,  not  inherently  im- 
probable or  absurd,  as  a  working  hypothesis,  a  point  of 
departure  for  further  experiments,  serving  to  orient  a 
whole  body  of  investigators,  seems  to  me  entirely  to  miss 
the  point  of  the  purpose  of  a  hypothesis.  Hypotheses 
are  not  statements  of  truth,  but  instruments  to  be  used 
in  the  ascertainment  of  truth.  Their  value  does  not  de- 

3Kellogg,  V.  L.,  Darwinism  To-day,    p.  223.    New  York,  1907. 


Translator's  Preface  vii 

pend  upon  ultimate  verification,  but  is  to  be  measured 
by  their  effect  upon  scientific  research.  All  this  is  now 
a  truism. 

What  does  it  argue  that  no  one,  as  Delage  insists, 
ever  anticipated  by  imagination  the  striation  of  muscle 
fibers,  the  existence  of  chromosomes  and  centrosomes,  or 
any  other  fact  of  minute  structure  revealed  by  the  micro- 
scope. May  it  not  be  asked  in  reply  how  long  we  should 
have  had  to  wait  for  the  discovery  of  the  inert  gases  of 
the  atmosphere,  the  accessory  chromosome,  and  the  ion, 
had  they  not  first  been  conceived  in  imagination  and 
formally  embodied  in  working  hypotheses?  It  is  not 
pleasant  to  contemplate  what  the  effect  on  the  develop- 
ment of  chemical  science  would  have  been  had  Dalton's 
(micromeric)  hypothesis  of  indivisible  units  been  rejected 
on  the  a  priori  grounds  that  the  ultimate  structure  of 
matter  is  beyond  the  power  of  the  human  intellect  to 
imagine  in  detail. 

The  hypothesis  of  intracellular  pangenesis  can  never 
be  absolutely  demonstrated  as  true — can  never  advance 
beyond  the  rank  of  a  theory — because  the  hypothetical 
pangens  are  conceived  to  be  invisible,  ultra-microscopic 
units,  whose  existence  can  never  be  more  than  inferred; 
but  the  formulation  of  the  hypothesis  marks  the  beginning 
of  the  greatest  and  most  important  forward  step  in  the 
study  of  the  origin  of  species  since  1859.  The  notion 
of  pangens  became  the  parent-idea  of  unit-characters, 
offered  a  simple  mechanism  for  the  disjunction  of  char- 
acters in  hybrids,  and  for  continuous  and  discontinuous 
variation,  and  thus  lead  up  directly  to  the  conception  of 
mutation  as  one  method  of  the  origin  of  species.4  And, 
most  important  and  significant  of  all,  it  resulted  in  per- 

4Cf.  footnote,  p.  74  infra. 


viii  Translator's  Preface 

manently  removing  the  entire  question  of  organic  evolu- 
tion from  the  realm  of  ineffective  speculation,  and  estab- 
lishing it  upon  the  firm  basis  of  experimentation. 

The  term  pangen  is  employed  in  its  original  sense  by 
Strasburger  in  his  paper  on  "Typische  und  allotypische 
Kertheilung."5 

Recognizing  the  existence  of  some  material  entities 
as  the  ultimate  units  of  heredity,  conceiving  of  them  as 
invisible,  and  accepting  for  them  the  name  pangen,  he 
interprets  the  chromatin  granules  (chromomeres),  which 
can  be  directly  seen,  as  larger  or  smaller  pangen-com- 
plexes,  and  suggests  that  we  designate  them  "pangeno- 
somes."  The  pangenosomes,  owing  to  a  "certain  elective 
affinity,"  he  considers  as  combining  into  ids,  (from  the 
idioplasm,  of  Nageli),  and  the  ids,  in  turn,  into  chromo- 
somes.6 

Referring  to  de  Vries's  supposition,  that  the  pangens 
influence  the  cytoplasm  by  wandering  out  into  it  from  the 
nucleus  and  thus  changing  from  an  inactive  to  an  active 
state,  Strasburger7  records  his  failure  to  detect  any  visi- 
ble evidence  that  the  bodies  which  he  calls  pangens  thus 
wander  out  from  the  nucleus  into  the  cytoplasm,  but  refers 
to  the  period  in  cell-division  when  the  nuclear  membrane 
disappears  and  the  spindle  forms,  as  serving  to  bring  the 
chromosomes  into  direct  contact  with  the  cytoplasm,  and 
thus  establishing  a  condition  favorable  for  the  ''forma- 
tive influencing"  of  the  cytoplasts  by  the  nucleoplasts.  A 
similar  influence  might  also  result  from  extranuclear  nu- 
cleoli  distributed  in  the  cytoplasm.  In  the  fertilization 

5Jahrb.  Wiss.  Bot.  42:  1-71.      1905. 

6Mottier's  use  of  the  word  pangen  to  designate  the  visible  chro- 
momeres (Ann.  Bot.  21;  307-347.  1907.),  employs  the  term  in  a 
sense  entirely  at  variance  with  that  for  which  it  was  originally  pro- 
posed (cf.  p.  49.) 

7hc.  cit.    p.  74. 


Translator's  Preface  ix 

of  the  egg  he  postulates  a  fusion  of  maternal  with  pater- 
nal pangens.8  Thus,  in  the  gametophytic  generation,  the 
pangens  must  be  considered  as  univalent  (haploid),  in 
the  sporophytic  as  bivalent  (dlploid).  This  would  lead 
us  to  look  for  larger  nuclei  in  the  cells  of  the  sporophyte 
than  in  those  of  the  gametophyte.  This  hypothesis  was 
verified  in  a  number  of  plants,  widely  separated  system- 
atically. In  Taxus  baccata,  for  example,  the  nuclei  of 
the  prothallus  were  noticeably  smaller  than  those  of  the 
sporophyte:  and  in  nuclei  with  equally  marked  granula- 
tion, Strasburger  counted  fifty  granules  in  an  optical  sec- 
tion' of  the  nuclei  of  the  nucellus,  and  only  one-half  that 
number  in  the  nuclei  of  the  adjacent  prothallus. 

But  I  cite  this  paper  of  Strasburger's  chiefly  to  show 
how  the  hypothesis  of  intracellular  pangenesis,  in  other 
hands  that  its  author's,  may  assist  in  forming  some  com- 
prehensible picture  of  the  mechanism  of  matter  in  the 
living  state.  The  idea  and  the  term  pangen  are  also 
adopted  by  Pfeffer  in  his  Physiology  of  Plants.9 

At  the  suggestion  of  Professor  de  Vries,  a  transla- 
tion of  his  Haarlem  Vortrag  on  "Befruchtung  und  Bas- 
tardierung"  is  included  in  this  volume,  for  the  purpose  of 
showing  the  bearing  of  more  recent  research  on  the  hy- 
pothesis of  intracellular  pangenesis,  and  of  thus  bringing 
the  problem  more  nearly  down  to  date.  The  translation 
of  this.  Vortrag  also  appeared  in  "The  Monist,"  for  No- 
vember, 1909. 

It  is  a  pleasure  to  record  my  profound  gratitude  to 
Professor  de  Vries  for  his  careful  reading  and  annota- 
tion of  the  manuscript  of  the  translation,  and  for  his  inter- 
est and  encouragement  throughout  the  undertaking. 

*loc.  cit.    p.  61. 

9Pfeffer  W.  The  Physiology  of  Plants.  Eng.  Trans,  by  Alfred 
J.  Ewart.  1:  49.  Oxford,  1900. 


x  Translator's  Preface 

I  am  deeply  indebted  to  Professor  Strasburger  for  his 
kindness  in  preparing  an  introductory  note,  and  wish,  also 
to  express  my  sincere  thanks  to  Miss  Marie  Onuf,  whose 
invaluable  assistance  rendered  the  completion  of  the  work 
possible. 

C.  S.  G. 

University  of  Missouri, 

Department  of  Botany. 

Nov.  13,  1909. 


Verlag  von  Gustav  Fischer  in  Jena 

Im  Jahre  1889  erschien  in  deutscher  Sprache  : 

INTRACELLULARE  PANGENESIS 

von 
HUGO  DE  VRIES 

Prof,  der  Botanik  a.  d.  Universitat  Amsterdam 

Preis:  4  Mark 


XI 


TABLE  OF  CONTENTS 
INTRACELLULAR   PANGENESIS 

PAGE 

AUTHOR'S  INTRODUCTION   3 

PART  I 

PANGENESIS 

A.  THE  NATURE  OF  HEREDITARY  CHARACTERS. 

CHAPTER  I.  THE  MUTUAL  INDEPENDENCE  OF  HEREDITARY  CHARACTERS. 

§1.  The  Combination  of  Specific  Characters  Out  of 

Hereditary  Characters  11 

§2.  The  Similarity  of  the  Differences  Between  Species 

and  Between  Organs  15 

§3.  The  Similarity  Between  Secondary  Sexual  Characters 

and  Specific  Attributes  18 

§4.  The  Variation  of  the  Individual  Hereditary  Char- 
acters Independently  of  One  Another 19 

§5.    The  Combination  of  Hereditary  Characters 24 

§6.    Cross-  and  Self-Fertilization   29 

§7.     Conclusion     33 

B.  PREVAILING  VIEWS   ON   THE   BEARERS   OF   HEREDITARY 

CHARACTERS. 

CHAPTER  II.  THE  SIGNIFICANCE  OF  THE  CHEMICAL  MOLECULES 
OF  THE  PROTOPLASM  WITH  REFERENCE  TO  THE 
THEORY  OF  HEREDITY. 

§1.     Introduction    37 

§2.     Protoplasm  and  Protein    41 

§3.     Elsberg's   Plastidules    44 

CHAPTER  III.    THE  HYPOTHETICAL  BEARERS  OF  SPECIFIC  CHARACTERS. 

§4.    Introduction    50 

§5.     Spencer's  Physiological  Units    51 

§6.    Weismann's  Ancestral  Plasms    53 

§7.     Nageli's    Idioplasm    57 

§8.     General  Considerations    59 


xii  Contents 

CHAPTER  IV.    THE  HYPOTHETICAL  BEARERS  OF  THE  INDIVIDUAL 
HEREDITARY  CHARACTERS. 

PAGE 

§9.    Introduction    62 

§10.     Darwin's  Pangenesis  63 

§11.     Critical   Considerations    66 

§12.     Conclusion     69 


PART  II 

INTRACELLULAR  PANGENESIS 

A.  CELLULAR  PEDIGREES. 

CHAPTER  I.    THE  RESOLVING  OF  INDIVIDUALS  INTO  THE  PEDIGREES 
OF  THEIR  CELLS. 

§1.  Purpose  and  Method   79 

§2.  The  Cellular  Pedigrees  of  the  Homoplastids 82 

§3.  The  Cellular  Pedigree  of  Equisetum 83 

§4.  The  Main  Lines  in  the  Cell-Pedigrees 88 

CHAPTER  II.      SPECIAL  CONSIDERATION  OF  THE  INDIVIDUAL  TRACKS. 

§5.    The  Primary  Germ-Tracks  93 

§6.     The  Secondary  Germ-Tracks 95 

.  §7.    The  Somatic  Tracks 100 

§8.     The  Difference  Between  Somatic  Tracks  and  Germ 

Tracks     103 

§9.     Phyletic,  Somatarchic,  and  Somatic  Cell-Divisions..   107 

CHAPTER  III.    WEISMANN'S  THEORY  OF  THE  GERM-PLASM. 

§10.     The   Significance  of  the  Cell-Pedigree  for  the  Doc- 
trine of  the  Germ-Plasm 110 

§11.    The  Views  of  Botanists   113 

§12.    A  Decision  Reached  Through  the  Study  of  Galls. ...   118 

B.  PANMERISTIC  CELL-DIVISION. 

CHAPTER  I.    THE  ORGANIZATION  OF  THE  PROTOPLASTS. 

§1.     The  Visible  Organization    125 

CHAPTER  II.    HISTORICAL  AND  CRITICAL  CONSIDERATIONS. 

§2.    The  Neogenetic  and  the  Panmeristic  Conception  of 

Cell-Division    128 

§3.     Cell-Division  According  to  Mohl's  Type   134 

§4.    The  Regeneration  of  Protoplasts  After  Wounding. .  139 


Contents  xiii 

PAGE 

CHAPTER  III.     THE  AUTONOMY  OF  THE  INDIVIDUAL  ORGANS  OF 
THE  PROTOPLASTS. 

§5.  Nucleus  and  Trophoplast 144 

§6.  The  Vacuoles  ' ISO 

§7.  The  Relation  Between  the  Plasmatic  Membranes  and 

the  Granular  Plasm 157 

§8.  The  Question  of  the  Autonomy  of  the  Limiting 

Membrane     160 


C.  THE  FUNCTIONS  OF  THE  NUCLEI. 

CHAPTER  I.    FERTILIZATION. 

§1.     Historical  Introduction  169 

CHAPTER  II.    FERTILIZATION  (continued). 

§2.    The  Conjugation  of  the  Zygosporeae 171 

§3.     Fertilization  in  Cryptogams    173 

§4.     Fertilization  in  Phanerogams  176 

CHAPTER  III.    THE  TRANSMISSION  OF  HEREDITARY  CHARACTERS 

FROM    THE    NUCLEI    TO    THE    OTHER    ORGANS    OF   THE 

PROTOPLASTS. 

§5.     The  Hypothesis  of  Transmission  179 

§6.     Observations  on  the  Influence  of  the  Nucleus  in  the 

Cell    - 183 

D.  THE  HYPOTHESIS  OF  INTRACELLULAR  PANGENESIS. 
CHAPTER  I.    PANGENS  IN  THE  NUCLEUS  AND  CYTOPLASM. 

§1.     Introduction    193 

§2.     AH  Protoplasm  Composed  of  Pangens 195 

§3.     Active  and  Inactive  Pangens   199 

§4.    The  Transportation  of  Pangens   201 

§5.  Comparison  with  Darwin's  Transportation-Hypothesis  207 

§6.    The  Multiplication  of  Pangens   212 

CHAPTER  II.    SUMMARY 

§7.     Summary  of  the  Hypothesis  of  Intracellular  Pange- 

nesis    215 


FERTILIZATION  AND  HYBRIDIZATION 
Fertilization  and  Hybridization    219 


AUTHOR'S  INTRODUCTION 


AUTHOR'S  INTRODUCTION 

In  the  year  1868,  in  the  second  volume  of  his  cele- 
brated work,  "The  variation  of  animals  and  plants  under 
domestication,"  Darwin  formulated  the  provisional  hypo- 
thesis of  pangenesis.  The  discussion  of  this  hypothesis 
is  preceded  by  a  masterly  survey  of  the  phenomena  to  be 
explained.  Owing  to  this,  as  well  as  to  his  clear  concep- 
tion of  the  whole  problem,  this  part  of  his  book  has  at- 
tracted universal  attention.  We  find  it  mentioned  in 
almost  all  works  which  deal  with  general  biological  ques- 
tions. While,  however,  the  general  part  of  the  chapter 
has  until  now  remained  the  basis  for  all  scientific  consid- 
erations of  the  nature  of  heredity,  the  hypothesis  itself 
has  not  enjoyed  such  general  appreciation. 

Darwin  assumes  (Variation  2:  369)  that  the  cells, 
as  is  generally  accepted,  multiply  by  division,  and  that  in 
so  doing  they  preserve  essentially  the  same  nature.  He 
considers  that  this  rule  forms  the  basis  of  heredity.  By 
it,  however,  not  all  of  the  groups  of  phenomena  brought 
together  by  Darwin  may  be  explained.  Especially  does 
it  not  explain  the  effects  of  use  and  disuse,  the  direct  ac- 
tion of  the  male  element  on  the  female,  and  the  nature  of 
graft-hybrids.  In  order  to  take  into  account  these  phe- 
nomena, Darwin  assumes  that  there  exists,  in  addition  to 
cell  division,  yet  another  means  of  transfer  of  hereditary 
qualities.  Each  unit  of  the  body,  according  to  his  theory, 


4  Author's  Introduction 

throws  off  minute  granules1  which  accumulate  in  the  germ 
cells  and  buds.  These  granules  are  the  bearers  of  the 
characters  of  the  cells  from  which  they  are  derived,  and 
thus  transmit  those  characters  to  the  germ  cells  and  to  the 
buds. 

Thus  all  the  hereditary  characters  of  the  organism 
are  represented  in  the  egg-cells,  pollen-grains,  sperm- 
cells,  and  buds  by  minute  particles.  These  they  have  re- 
ceived, partly  by  descent  from  former  germ  cells,  i.  e., 
directly,  but  partly  by  later  addition  from  the  cells  and 
organs  of  the  body.  These  minute  granules  are  not  the 
chemical  molecules;  they  are  much  larger  than  these  and 
are  more  correctly  to  be  compared  with  the  smallest 
known  organisms.  Darwin  calls  them  gemmules  (small 
germs).  * 

The  hypothesis  of  these  gemmules  threw  an  unex- 
pected light  on  a  series  of  facts  which  had  hitherto  been 
in  absolute  darkness.  And  if  one  reads  attentively  Dar- 
win's discussion,  he  sees  more  and  more  clearly  that  the 
transmission  of  gemmules  by  cell-division,  from  the 
mother-cell  to  the  daughter-cell,  suffices  to  explain  large 
groups  of  phenomena.  Only  isolated  groups  of  facts  de- 
mand in  addition  the  hypothesis  of  transportation.  The 
doctrine  of  latent  qualities  and  of  atavism  particularly 
are  drawn  from  their  former  darkness  by  Darwin's  hy- 
pothesis, and  his  discussion  of  this  subject  (p.  357) 
clearly  shows  what  great  significance  he  imputes  to  this 
circumstance.  It  demands,  however,  only  the  transmis- 
sion of  the  gemmules  in  cell-division,  not  their  transpor- 
tation from  the  growing  and  full-grown  organs  to  the 
germ-cells. 

1This  is  the  term  Darwin  first  uses.    The  Variation  of  Animals 
and  Plants.  2:  358.     New  York,  1900.     Tr. 


Author's  Introduction  5 

It  has  always  seemed  to  me  that  most  authors  have  not 
sufficiently  distinguished  these  two  aspects  of  the  hy- 
pothesis, and  that  their  objections  against  accepting  the 
theory  of  transportation  have  misled  them  into  over- 
looking the  paramount  significance  of  the  doctrine  of 
gemmules.  . 

To  my  mind  Darwin's  provisional  hypothesis  of  pan- 
genesis  consists  of  the  following  two  propositions : 

1.  In  every  germ-cell   (egg-cell,  pollen-grain,  bud, 
etc.)  the  individual  hereditary  qualities  of  the  whole  or- 
ganism  are   represented   by   definite   material   particles. 
These  multiply  by  division  and  are  transmitted  during 
cell-division  from  the  mother-cell  to  the  daughter-cells. 

2.  In  addition,  all  the  cells  of  the  body,  at  different 
stages  of  their  development,  throw  off  such  particles; 
these  flow  into  the  germ-cells,  and  transmit  to  them  the 
qualities  of  the  organism,  which  they  are  possibly  lack- 
ing.    (  Transportation-hypothesis  ) . 

The  second  assumption  possessed,  for  Darwin  himself, 
only  limited  importance,  in  the  case  of  plants  and  corals, 
as  he  considered  a  transportation  of  gemmules  from  one 
branch  to  another  impossible.  It  does  not  apply  to  the 
workers  of  ants  and  bees,  nor  to  the  double  stocks  (gilli- 
flower)  mentioned  several  times  by  Darwin.  These  do 
not  possess  any  stamens  and  pistils  themselves,  and  their 
characteristics  must  therefore  be  transmitted  from  one 
generation  to  the  other  through  the  fertile  single  specimens 
of  the  race.  The  facts,  for  the  explanation  of  which  the 
theory  in  question  was  brought  forth,  have  gained  neither 
in  number  nor  in  trustworthiness  during  the  twenty  years 
since  the  publication  of  Darwin's  book. 

Doubts  of  its  necessity,  therefore,  are  quite  permis- 
sible, and  it  is  the  chief  service  of  Weismann  to  have 


6  Author's  Introduction 

repeatedly  emphasized  these  doubts,  and  to  have  shat- 
tered the  rather  generally  accepted  doctrine  of  the  hered- 
ity of  acquired  characters.2 

But  even  if,  with  this  investigator,  one  rejects  the 
second  proposition,  that  is  no  reason  for  likewise  doubt- 
ing the  other  part  of  the  hypothesis  of  pangenesis.  On 
the  contrary,  it  seems  to  me  that  by  doing  so  its  great 
significance  only  becomes  clearer.  Besides,  there  have 
been  no  convincing  arguments  brought  forward  against 
this  first  dogma,  and  no  other  hypothesis  concerning  the 
nature  of  heredity  takes  account  of  the  facts  in  so  simple 
and  clear  a  manner. 

Yet  most  authors  have  considered  that,  by  refuting 
the  transportation  hypothesis,  they  have  also  refuted  that 
of  the  bearers  of  individual  hereditary  characters,  and 
they  have  hardly  devoted  any  special  discussion  to  it.  In 
consequence  of  this  Darwin's  view  has  unfortunately  not 
borne  such  fruit  for  the  development  of  our  knowledge 
as  its  originator  had  a  full  right  to  expect. 

My  problem  in  the  following  pages  will  be  to  work 
out  the  fundamental  thought  of  pangenesis  independently 
of  the  transportation  hypothesis,  and  to  connect  with  it 
the  new  facts  which  the  doctrine  of  fertilization  and  the 
anatomy  of  the  cell  have  brought  to  light. 

I  shall  be  guided  by  the  thought  that  the  physiology 
of  heredity,  and  especially  the  facts  of  variation  and  of 
atavism  indicate  the  phenomena  which  are  to  be  explained, 
while  microscopic  investigation  of  cell-division  and  fer- 
tilization will  teach  us  the  morphological  substratum  of 
those  processes.  We  shall  not  try  to  explain  the  mor- 

2The  designation  "acquired"  is  not  exactly  well  chosen.  The 
question  is:  Can  characters  which  have  originated  in  somatic  cells 
be  communicated  to  the  germ -cells.  This  possibility  is  rejected  by 
Weismann.  Compare  Part  II,  §  5.  (p.  93). 


Author's  Introduction  7 

phological  details  of  those  processes;  our  knowledge  is 
yet  too  limited  for  that.  But,  following  the  method  of 
Darwin,  to  find  in  the  special  cases  the  material  substra- 
tum of  the  physiological  processes,  that  is  our  problem. 

As  the  most  important  result  of  cell-investigation  of 
the  preceding  decades,  I  consider  the  theory  that  all  the 
hereditary  predispositions  (Anlagen)  of  the  organism 
must  be  represented  in  the  nucleus  of  the  cell.  I  shall  try 
to  show  that  this  theory  leads  us  to  assume  a  transporta- 
tion of  material  particles  which  are  bearers  of  the  indi- 
vidual hereditary  characters.  This  does  not  mean,  how- 
ever, a  transportation  through  the  whole  organism,  nor 
even  from  one  cell  to  another,  but  one  restricted  to  the 
limits  of  the  individual  cells.  From  the  nucleus  the  ma- 
terial bearers  of  the  hereditary  characters  are  transported 
to  the  other  organs  of  the  protoplast.  In  the  nucleus  they 
are  generally  inactive,  in  the  other  organs  of  the  protoplast 
they  may  become  active.  In  the  nucleus  all  characters 
are  represented,  in  the  protoplast  of  every  cell  only  a 
limited  number. 

The  hypothesis,  therefore,  becomes  one  of  intracellu- 
lar  pangenesis.  To  the  smallest  particles,  of  which  each 
represents  one  hereditary  characteristic,  I  shall  give  a 
new  name  and  call  them  pangens,  because  with  the  desig- 
nation "gemmule"  (Keimchen)  is  associated  the  idea  of  a 
transportation  through  the  whole  organism. 


PARTI 

PANGENESIS 
A.     THE  NATURE  OF  HEREDITARY  CHARACTERS 


CHAPTER  I 

THE    MUTUAL   INDEPENDENCE   OF    HEREDITARY 
CHARACTERS 

§    /.    The   Combination  of   Specific   Characters   Out   of 
Hereditary  Characters 

Among  the  many  advantages  which  have  lent  such 
a  prominent  significance  to  the  theory  of  descent  in  the 
investigation  of  living  nature,  the  shattering  of  the  old 
conception  of  species  occupies  an  important  place.  For- 
merly every  species  was  regarded  as  a  unit  and  the  total- 
ity of  its  specific  attributes  as  an  indivisible  concept.  Even 
the  latest  theories  on  heredity  accept  this  concept  as  one 
that  does  not  require  any  further  analysis. 

But  if  the  specific  characters  are  regarded  in  the  light 
of  the  theory  of  descent  it  soon  becomes  evident  that  they 
are  composed  of  single  factors  more  or  less  independent  < 
of  each  other.    One  finds  almost  every  one  of  these  fac- 
tors in  numerous  species,  and  their  varying  groupings^- 
and  combinations  with  less  common  factors  causes  the 
extraordinary  diversity  in  the  organic  world. 

Even  the  most  cursory  comparison  of  the  various  or- 
ganisms leads,  in  this  light,  to  the  conviction  of  the 
composite  nature  of  specific  characters.  The  power  to 
produce  chlorophyll  and,  by  means  of  this,  in  light,  to 
decompose  carbon  dioxide,  is  evidently  to  be  regarded  as 
a  property  which,  in  great  measure  lends  to  the  botanical 
world  its  peculiar  stamp.  This  power,  however,  is  lack- 
ing in  many  groups  throughout  the  system,  and  therefore 


12    Mutual  Independence  of  Hereditary  Characters 

is  by  no  means  inseparably  connected  with  the  other  fac- 
tors of  plant  nature. 

Other  factors  are  the  predispositions  (Anlagen) 
which  enable  many  species  to  produce  definite  chemical 
compounds.  First  of  all,  the  red  and  blue  coloring  mat- 
ter of  flowers,  then  the  different  tannic  acids,  the  alka- 
loids, etherial  oils,  and  numerous  other  products.  Only 
a  few  of  these  are  limited  to  a  single  species,  many  recur 
in  two  or  more  species,  which  are  often  systematically  far 
apart.  There  is  no  reason  for  supposing  that,  in  every 
individual  case  there  is  a  different  mode  of  origin  for 
the  same  compound;  rather  it  is  obvious  that  essentially 
the  same  chemical  mechanism  underlies  the  same  process, 
wherever  we  find  it. 

In  a  similar  manner  we  must  also  accept  as  possible 
the  analysis  of  the  morphological  characteristics  of  the 
species.  It  is  true  that  morphology  is  not  by  any  means 
so  far  advanced  that  such  an  analysis  could  be  carried  out 
in  every  individual  case.  But  the  same  leaf-form,  the 
same  leaf-edge,  coarsely  or  delicately  notched,  recur  in 
numerous  species,  and  even  the  customary  terminology 
teaches  us  that  the  configurations  of  all  the  various  leaf- 
forms  are  composed  of  a  comparatively  small  number  of 
simple  characters. 

It  would  be  superfluous  to  accumulate  instances  which 
are  easily  accessible  to  every  one,  and  it  is  only  a  question 
of  thoroughly  familiarizing  one's  self  with  these  ideas,  so 
that  the  synthesis  of  the  whole  out  of  its  component  parts 
is  clearly  recognized.  It  will  then  be  seen  that  the  character 
of  each  individual  species  is  composed  of  numerous  hered- 
itary qualities,  of  which  by  far  the  most  recur  in  almost 
innumerable  other  species.  And  even  if,  in  the  building 
up  of  any  single  species,  such  a  large  number  of  these 


Specific  Characters    Composite  13 

factors  is  necessary  that  we  almost  shrink  from  the  con- 
sequences of  an  analysis,  it  is  clear,  on  the  other  hand, 
that,  for  the  building  up  of  the  sum  total  of  all  organisms, 
there  is  required  a  rather  small  number  of  individual 
hereditary  characters  in  proportion  to  the  number  of 
species.  Regarded  in  this  way,  each  species  appears  to  us  ^ 
as  a  very  complex  picture,  whereas  the  whole  organic 
world  is  the  result  of  innumerable  different  combinations 
and  permutations  of  relatively  few  factors. 

These  factors  are  the  units  which  the  science  of  hered- 
ity has  to  investigate.  Just  as  physics  and  chemistry  go 
back  to  molecules  and  atoms,  the  biological  sciences  have 
to  penetrate  to  these  units  in  order  to  explain,  by  means 
of  their  combinations,  the  phenomena  of  the  living  world. 

Phylogenetic  considerations  lead  to  the  same  conclu- 
sions. Species  have  gradually  been  evolved  from  simpler 
forms,  and  this  has  taken  place  by  the  addition  of  more 
and  more  new  characteristics  to  those  already  existing.  \ 
The  factors  which  compose  the  character  of  a  single  spe- 
cies are,  therefore,  in  this  sense,  of  unequal  age ;  the  char- 
acteristics of  the  larger  groups  being  in  general,  older 
than  those  of  the  smaller  systematic  divisions.  But  the 
very  consideration  that  the  characteristics  have  been  ac- 
quired singly  or  in  small  groups,  shows  us  again  from 
another  side  their  mutual  independence. 

It  is  a  striking,  yet  by  far  insufficiently  appreciated 
fact  that  frequently,  in  distant  parts  of  the  genealogical      / 
tree,  the  same  character  has  been  developed  by  wholly^ 
different  species.      Such  "parallel  adaptations"  are  ex- 
tremely numerous,  and  almost  every  comparative  treat- 
ment   of    a    biological    peculiarity    shows    us    examples 
thereof.     The  insect-eating  plants  belong  to  the  most 
varied  natural  families,  yet  they  all  possess  the  power  of 


14    Mutual  Independence  of  Hereditary  Characters 

producing  from  their  leaves  the  necessary  mixture  of  an 
enzyme,  and  of  an  acid  which  is  needed  for  dissolving 
protein  bodies.1  The  agreement,  emphasized  by  Darwin, 
of  this  mixture  with  the  gastric  juice  of  the  higher  ani- 
mals justifies  even  the  supposition  that  those  plants  and 
the  animal  kingdom  have  some  hereditary  qualities  in 
common. 

The  indigenous  creeping  and  climbing  plants,  the  trop- 
ical lianas,  the  tuberous  and  bulbous  plants,  the  fleshy, 
leafless  stems  of  the  Cactaceae  and  Euphorbiaceae,  the 
pollinia  of  the  Orchidaceae  and  Asclepiadaceae,  and  num- 
berless other  instances  show  us  parallel  adaptations.  Very 
beautiful  pictures  are  furnished  on  the  one  hand  by  the 
desert  plants,  which  all  have  to  protect  themselves  in  some 
way  against  the  disadvantages  of  evaporation,  and  whose 
anatomical  relations  have  been  so  thoroughly  described 
by  Volkens.2  On  the  other  hand  are  the  ant-plants,  into 
the  adaptations  of  which  to  harmful  and  useful  species  of 
ants  Schimper  has  given  us  an  insight.3 

Everywhere  we  see  how  one  and  the  same  hereditary 
character,  or  definite  small  groups  of  the  latter,  can  com- 
bine with  other  most  diverse  hereditary  characters,  and 
how,  through  these  exceedingly  varied  combinations,  the 
individual  specific  characters  are  produced. 

1This  statement  is  now  known  to  hold  true  only  in  the  case  of 
Nepenthes  (Vines,  Ann.  Bot.  11:  563.  1897.  12:  545.  1898)  and  of 
Drosera  (see  Fr.  Darwin's  articles).  Schimper  found  no  proteolytic 
enzyme  secreted  by  Sarracenias.  (Bot.  Zeit.  40:  225.  1882).  His 
results  were  confirmed  by  Miss  Robinson,  but  she  demonstrated  the 
secretion,  by  Sarracenia  purpurea,  of  a  starch-digesting  enzyme. 
(Torreya  8:  1908).  Tr. 

2Volkens,  G.    Die  Flora  der  Aegyptisch-Arabischen  Wuste. 

3Schimper,  A.F.W.  Die  Wechselbeziehungen  zwischen  Pflanzen 
und  Ameisen  im  tropischen  Amerika.  Bot.  Mittheil.  aus  den  Tropen. 
Band  I,  Heft  1,  1888. 


Organic  Characters  Composite  15 

§  2.  The  Similarity  of  the  Differences  Betzveen  Species 
and  Between  Organs 

The  comparison  of  species  with  the  organs  of  a  single 
individual  leads  us  to  quite  similar  conclusions  as  does 
the  comparison  of  species  with  each  other,  for  the  dif- 
ferences between  the  organs  can  be  traced  back,  in  the 
same  way,  to  various  combinations  of  individual  heredi- 
tary qualities. 

Even  the  simplest  observation  teaches  us  this.  Just 
as  chlorophyll  is  lacking  in  some  species  it  is  also  lacking 
in  single  organs  and  tissues  of  higher  plants.  The  red 
coloring  matter  of  flowers  is  limited  to  certain  plant 
species,  and  in  these  again  to  definite  organs.  Tannic 
acid,  etherial  oils,  and  like  substances,  where  present, 
show  a  local  distribution.  Calcium  oxalate  is  lacking  in 
most  ferns  and  grasses,  and  on  the  other  hand  in  the  roots 
of  many  species  rich  in  calcium.  The  same  is  true,  ap- 
parently, of  morphological  attributes.  I  need  not  cite  ex- 
amples, for  it  will  certainly  be  granted  that  a  very  close 
agreement  exists  between  the  manner  in  which  the  or- 
gans of  a  single  plant  differ  from  each  other  and  the  dis- 
tinction between  different  species.  Both  depend  upon 
varying  combinations  and  a  varying  selection  from  a 
great  range  of  given  factors. 

A  series  of  phenomena,  which  we  may  summarize  un- 
der the  name  dichogeny,  leads  to  similar  conclusions.  I 
mean  all  those  cases  where  the  nature  of  an  organ  is  not 
yet  decided  during  the  early  stages  of  its  development, 
but  may  yet  be  determined  by  external  influences.  Thus, 
under  normal  conditions,  the  runners  of  the  potato-plant 
form  at  their  tips  the  tubers,  but  on  being  exposed  to 
light,  or  when  the  main  stem  has  been  cut  off,  they  de- 


16    Mutual  Independence  of  Hereditary  Characters 

velop  into  green  shoots.  By  severing  the  stems,  the 
rhizomes  of  Mentha,  Circaea,  and  many  other  plants,  can 
be  made  into  ascending  stems,  and  the  transformations 
which  the  thick  almost  resting  rhizomes  of  Yucca  undergo 
after  such  treatment  are  remarkable.  In  a  similar  manner 
Goebel  has  succeeded  in  causing  the  rudiments  of  bracts 
to  develop  into  green  leaves,4  and  Beyerinck5  observed 
even  the  transformation  of  young  buds  of  Rumex  Aceto- 
sella  into  roots. 

In  such  cases  it  is  clear  that  the  possibility  of  develop- 
ing in  either  of  two  different  directions  is  dormant  in  the 
young  primordia.  For  this  very  reason  I  should  like  to  ap- 
ply the  name  dichogeny  to  this  phenomenon.  And  it  evi- 
dently depends  upon  external  influences  what  direction  is 
taken.  Therefore  a  selection  must  take  place  from  among 
the  available  hereditary  characters  of  the  species,  and  this 
selection  may  be  influenced  by  artificial  interference.  For 
the  theory  of  hereditary  characters  such  experiments  are 
therefore  of  the  highest  interest. 

Here  are  naturally  included  the  phenomena  of  bud- 
variation.  Many  of  these  are  cases  of  atavism.  Let  us 
select  an  example.  In  plants  with  variegated  leaves  one 
frequently  observes  single  green  branches.  Since  the 
variegated  plant  is  descended  from  green  ancestors,  this 
case  is  regarded  as  a  reversion.  The  variegated  individual 
evidently  still  possessed  the  characteristics  of  the  green 
ancestor,  though  in  a  latent  condition.  During  the  bud- 
formation  it  split  its  entire  character,  but  in  such  a  way 

4Goebel,  K.  Beitrage  zur  Morphologic  und  Physiologic  des 
Blattes.  Bot.  Zeit.  40:  353.  1882. 

5Beyerinck,  M.  W.  Beobachtungen  und  Betrachtungen  iiber 
Wurzelknospen  und  Nebenwurzeln.  Veroffentl.  Akad.  Wiss.  Am- 
sterdam, pp.  41-41.  1886.  Cf.  also  Tafel  T,  Fg.  9. 


Bud-  Variation  1 7 

that  in  one  branch  the  variegated  combination  predomi- 
nated, in  the  other  one  the  green  color. 

As  a  further  illustration  of  bud-variation,  I  may  men- 
tion the  nectarines.  These  are  hairless  peaches,  which 
originated  in  several  varieties,  and  in  some  of  them  re- 
peatedly through  bud-variation.  This  fact  can  be  ex- 
plained only  by  saying  that  the  possibility  of  producing 
hairy  fruit  can  become  lost  in  single  branches,  easily  and 
independently  from  all  other  characters,  or  at  least  be- 
come latent. 

The  characteristics  which  originate  through  bud-vari- 
ation are  usually  preserved  by  propagation  by  means  of 
grafts,  cuttings,  et  cetera,  and,  in  isolated  cases,  are  even 
constant  from  seed.  New  varieties  may  therefore  be  pro- 
duced in  this  manner.  And,  since  we  regard  varieties  as 
incipient  species,  this  consideration  is  further  evidence  of 
an  accordance  in  the  differences  between  species  and  be- 
tween organs. 

Naturally  included  with  bud-variations  is  the  consid- 
eration of  monoecious  plants,  for  the  latter  agree  with  the 
former  in  the  fact  that  different  branches  allow  different 
qualities  to  develop.  In  the  young  plant  the  sexes  are  not 
yet  separated,  and  frequently  for  a  long  time  the  possibil- 
ity of  producing  both  is  retained.  If  this  process,  how- 
ever, is  started,  it  is  accomplished  by  a  kind  of  separation : 
one  bud  develops  into  a  staminate,  the  other  into  a  pistil- 
late flower.  Or  staminate  and  pistillate  inflorescences  are 
produced,  or  whole  branches  are  predominantly  pistillate 
and  others  staminate.  The  specific  character  was  there- 
fore present  in  the  young  plant  as  a  whole,  but  in  a  latent 
state,  and,  in  order  to  manifest  itself,  it  had  to  split  into 
its  two  chief  parts. 

The  formation  of  organs,  bud-variation,  and  the  pro- 


18    Mutual  Independence  of  Hereditary  Characters 

duction  of  staminate  and  pistillate  branches  in  monoecious 
plants  are  therefore  due  to  a  kind  of  splitting.  The  po- 
tentialities, united  in  the  young  plant,  separate  from  each 
other  in  order  to  be  able  to  unfold.  And  the  grouping 
of  the  hereditary  characters  in  the  separate  branches  and 
organs  shows  a  very  great  agreement  with  the  combina- 
tion of  such  characters  to  form  the  various  specific  marks 
of  related  organisms. 

§  j.  The  Similarity  Betzveen  Secondary  Sexual  Characters 
and  Specific  Attributes 

Continuing  in  a  similar  manner  as  in  the  previous 
paragraph  we  will  now  take  into  consideration  the  sec- 
ondary sexual  characters,  for  they  lead  to  exactly  the 
same  conception  of  a  specific  character. 

This  is  most  clearly  seen  in  those  cases  where  the  two 
sexes  of  one  species,  upon  being  first  discovered  have  been 
described  as  different  species.6  But  otherwise,  too,  the 
secondary  differences  between  the  individuals  of  both 
sexes  are  of  the  same  order  as  the  differences  between  the 
various  species  in  the  same  and  in  allied  genera. 

It  is  the  same  with  those  plants  which  bear  flowers  on 
various  individuals,  the  sex-organs  of  which  exhibit  con- 
stant differences,  the  so-called  cases  of  heterostyly.  In 
the  Primulaceae  we  distinguish  one  form  with  long  and 
another  with  short  style;  in  some  species  of  flax  there 
occur  three  different  forms  of  flowers  in  different  indi- 
viduals. 

Although  here  the  individuals  belonging  to  two  or 
three  different  groups  of  the  same  species  are  different 

6Catasetum  tridentatum  has  three  different  forms  of  flowers, 
which  were  formerly  considered  to  belong  to  three  different  genera : 
Catasetum,  Monachanthus  and  Myanthus.  de  V.,  1909. 


Variation  of  Individual  Hereditary  Characters     19 

neither  according  to  sex  nor  to  generation,  nevertheless 
they  are  distinguished  by  attributes  which  are  as  constant 
and  of  the  same  order  as  the  specific  attributes  taken  from 
the  same  organs  in  allied  genera. 

In  the  way  of  a  supplement  I  will  consider,  in  this 
connection,  the  alternation  of  generations,  because  here 
also  the  differences  between  the  physiologically  non- 
equivalent  individuals,  belonging  to  different  generations, 
are  of  the  same  order  as  the  specific  characters.  This  we 
are  taught  by  the  Uridineae  and  the  Cynipideae,  and  all 
those  cases  where  the  presence  of  an  alternation  of  gen- 
erations was  discovered  only  after  the  single  forms  had 
been  described  as  species,  and  had  been  classified  with  dif- 
ferent genera  and  families  of  the  system.  And  even  to- 
day it  is  impossible  to  prove  morphologically  that  two 
forms  belong  together;  experimental  cultures  alone  can 
decide  this  question.  The  successive  alternating  genera- 
tions cannot  be  reduced  to  the  same  primary  form,  for 
each  of  them  compounds  its  characters  by  means  of  a  dif- 
ferent selection  from  the  available  hereditary  endow- 
ments of  the  species. 

In  summing  up  the  result  of  this  paragraph  and  the 
two  preceding  ones,  we  find  that  every  thorough  consid- 
eration of  a  specific  character,  and  every  comparison  of 
this  with  other  characters,  leads  us  to  regard  the  former  as 
a  mosaic,  the  component  parts  of  which  can  be  put  to- 
gether in  various  ways. 

§  4.    The  Variation  of  the  Individual  Hereditary  Charac- 
ters Independently  of  One  Another 

A  comparative  consideration  of  the  organic  world 
convinced  us  that  the  hereditary  characters  of  a  species, 
even  if  connected  with  each  other  in  various  ways,  are 


20    Mutual  Independence  of  Hereditary  Characters 

yet  essentially  independent  entities,  from  the  union  of 
which  the  specific  characters  originate.  Now  let  us  see 
whether  or  not  this  conclusion  is  supported  by  experi- 
ment. 

For  this  purpose  let  us  turn  to  experiments  on  the 
formation  of  varieties,  especially  to  those  which  have  been 
made  on  a  large  scale  by  plant  breeders.  They  teach  us 
that  almost  every  character  may  vary  independently  from 
the  others.  Numerous  varieties  differ  from  their  ancestral 
form,  in  only  one  attribute,  as,  for  example,  the  white 
sports  of  red-flowered  species.  The  red  color  changes  in 
the  corolla  through  all  gradations,  into  white;  it  may  be 
lacking  or  it  may  be  present  not  only  in  the  blossoms,  but 
also  in  the  stems  and  leaves,  and  can  be  developed  to  every 
conceivable  degree,  without  any  other  hereditary  quality 
being  necessarily  involved  in  the  variation.  In  the  same 
way  the  hairine'ss,  the  arming  with  thorns  and  spines,  the 
green  color  of  the  leaves,  may  each  vary  by  itself,  and 
even  disappear  completely  while  all  other  hereditary  char- 
acters remain  quite  unchanged.  Frequently  some  charac- 
ters that  belong  together  vary  in  groups  without  exercising 
any  influence  on  the  other  groups.  Thus  an  increase  in 
the  number  of  petals  is  not  rarely  accompanied  by  a  petal- 
like  development  of  the  calyx  or  the  bract-leaves,  while 
otherwise  the  plant  remains  normal.  I  have  cultivated 
a  Dipsacus  sylvestris,  which  offers  all  conceivable  diver- 
sities in  the  arrangement  of  the  leaves,  and  which  is  other- 
wise constant  in  thousands  of  specimens.  The  Papaver 
sommferwn  polycephalum  deviates  only  in  the  transfor- 
mation of  numerous  stamens  into  carpels.  It  is  the  same 
for  the  cultivated  Sempervirum  tectorum.  Such  instances 
are  so  numerous,  in  the  plant  kingdom  as  well  as  in  the 
animal  kingdom,  that  the  independent  varying  of  single 


Influence  of  Environment  21 

characteristics  forms  the  rule,  while  the  combined  varia- 
tion of  several  of  them  is  the  exception.  It  is  true  that 
in  most  cases  it  cannot  be  decided  whether  the  given  at- 
tribute is  determined  by  a  single  hereditary  character  or 
by  a  small  group  of  them. 

On  the  other  hand  an  accumulation  of  several  varia- 
tions in  one  race  can  easily  be  accomplished,  and  it  occurs 
quite  commonly  in  cultures  as  well  as  in  nature.  But  the 
cases  which  were  sufficiently  well  controlled  and  de- 
scribed, usually  show  that  the  single  variations  have  not 
evolved  simultaneously,  but  one  after  another,  and  this 
is  sufficient  to  prove  their  independence. 

Such  an  hereditary  character,  isolated  from  the  rest, 
can  now  become  the  object  of  experimental  treatment. 
Through  suitable  selection  it  may  be  gradually  strength- 
ened or  weakened,  and  at  the  will  of  the  breeder  it  may 
be  brought  into  a  certain  relation  to  the  other  unchanged 
characters.  The  red  color  of  the  copper-beech  has  been 
so  much  intensified  that  even  the  cell-sap  in  the  living 
cells  of  the  wood  became  intensely  red.  The  doubling  of 
flowers  frequently  leads  to  a  complete  disappearance  of 
the  sexual  organs.  And  in  numerous  instances  only  those 
organs  change  which  are  subjected  to  selection  while  the 
others  remain  unaffected  by  it.  The  adaptation  of  the 
cultivated  plants  of  agriculture  to  the  needs  of  man,  and 
of  the  horticultural  ones  to  his  aesthetic  sense,  demon- 
strates this  to  us  in  the  clearest  manner. 

Experimental  treatment  further  leads  to  the  study  of 
the  influence  of  external  circumstances  on  the  unfolding 
of  hereditary  characters.  Here  again  these  prove  them- 
selves to  be  factors  of  which  each  may  vary  independ- 
ently from  the  others.  Young  varieties  especially  are 
objects  for  study,  and  all  those  which  have  not  as  yet 


22    Mutual  Independence  of  Hereditary  Characters 

been  sufficiently  fixed,  and  in  which,  therefore,  external 
influences  will  still  play  a  prominent  part  in  answering 
the  question  as  to  whether  a  given  seed  will  produce  a 
true  or  an  atavistic  individual.  Rimpau  and  others  have 
taught  that  with  a  given  kind  of  seed  disturbances  and 
interruptions  of  growth  exercise  a  powerful  influence  on 
the  number  of  specimens  that  bear  seed  in  the  first  year.7 
And  in  horticultural  and  teratological  literature  one  finds 
scattered  numerous  data  from  which  the  importance  of 
external  influences  generally  is  clearly  evident.  To  the 
experimental  investigator  there  is  here  opened  a  large  and 
almost  untrodden  field.  Theoretically  the  chief  task  will 
consist  in  isolating  as  much  as  possible  the  variations  of 
the  hereditary  characters  in  order  to  obtain,  in  this  way, 
a  knowledge  of  the  individual  factors  of  the  respective 
character. 

The  variations  which  we  observe  in  nature  frequently 
appear  to  us  as  if  they  had  suddenly  sprung  into  existence, 
ancl  the  same  is  true  of  cultures  on  a  small  scale,  or  when 
the  single  indivduals  are  not  completely  under  control. 

However,  experience  with  cultivated  plants,  during 
the  first  years  after  the  beginning  of  their  cultivation, 
teaches  us  that  the  deviations  often  develop  but  slowly, 
and  that  the  modifying  influences,  as  a  rule,  have  to 
^operate  through  several  generations  before  they  can  ac- 
cumulate their  effect  in  such  a  manner  that  it  becomes 
evident.8  The  facts  with  reference  to  this,  collected  by 
Darwin,  give  the  impression  that  the  new  characters  at 
first  arise  only  in  a  latent  state,  and  in  this  condition  grad- 

7Rimpati,  A.  W.  Das  Aufschiessen  der  Runkelruben  Land- 
wirtschaft.  Jahrbilcher.  9:  191.  1880. 

8On  this  point  compare  Darwin,  The  Variation  of  Animals  and 
Plants  under  Domestication.  Ed.  2.  2:  39.  1875. 


Atavism  23 

ually  gain  in  strength,  until  they  finally  reach  the  stage 
necessary  to  make  them  visible.  Here  again  it  must  there- 
fore be  assumed  that  every  hereditary  character  is  misci- 
ble  to  any  extent  with  the  others. 

The  independence  of  the  hereditary  characters  is  most 
beautifully  shown  in  atavism.  A  character  may  remain 
latent  through  a  number  of  generations  while  all  the 
others  unfold  normally.  From  time  to  time  it  appears 
again,  mostly  without  exercising  any  kind  of  influence  on 
the  other  characters.  We  do  not  know  what  external 
circumstances  condition  this  reappearance;  in  all  prob- 
ability they  do  not  act  simply  on  the  atavistic  individuals,  . 
but  we  must  conceive  that  the  given  potentiality  is  alway^ 
latent  in  the  others,  only  it  is  very  fluctuating  in  its 
strength.  To  us  only  the  crests  of  the  highest  waves  are 
visible. 

To  all  appearance  such  qualities  can  be  transmitted 
through  a  long  series  of  generations,  from  one  generation 
to  another.  Their  existence  can  be  reckoned  by  millen- 
niums in  those  cases  where  they  are  at  least  as  old  as  the 
species  itself.  I  mean  the  cases  of  reversion  to  the  ances- 
tors of  the  species,  of  which  the  zebra-like  stripes  of  the 
horse  form  such  a  well-known  instance.9  We  have  a 
similar  illustration  in  the  Primula  acaulis  var.  caulescens, 
which  occurs  from  time  to  time  in  the  field  as  a  quite 
isolated  specimen  among  thousands  of  non-umbellate 
plants,  and  then  forms  an  inflorescence  quite  similar  to 
that  of  the  most  nearly  allied  umbellate  species.  Culti- 
vation has  taken  possession  of  this  more  richly  flowering 
variety,  and  has  put  it  on  the  market  in  many  nuances  of 
color. 

I  should  not  close  this  section  without  having  pointed 

9Darwin,  Joe.  cit,  1:  59. 


24    Mutual  Independence  of  Hereditary  Characters 

out  one  phenomenon  which  greatly  complicates  the  study 
of  hereditary  characters.  I  refer  to  the  circumstance,  al- 
ready repeatedly  alluded  to,  of  their  being  commonly 
united  in  smaller  and  larger  groups  which  behave  like 
units,  the  single  members  of  the  groups  usually  appear- 
ing together.  We  see  this  in  the  staminate  and  pistillate 
flowers  and  inflorescences  of  monoecious  plants,  in  the 
described  cases  of  bud- variation  and  dichogeny.  The 
sexual  characters  of  various  individuals  and  the  differ- 
ence between  the  alternating  generations  of  the  same  spe- 
cies teach  us  the  same  thing. 

This  combination  of  the  individual  characters  into 
groups  is  therefore  quite  general,  although  it  occurs  in 
all  degrees,  and  although  some  hereditary  characters,  as 
for  instance  the  power  of  assuming  a  red  color,  do  not 
unite,  as  a  rule,  into  a  group  with  certain  others.  It  is 
recognized  most  clearly  in  those  cases  of  the  formation  of 
groups  of  green  bracts  instead  of  flowers,  caused  by 
aphids,  phytopters,  and  other  parasites,  where  the  stimu- 
lus calls  forth  a  whole  series  of  characters  that  ordinarily 
develop  in  other  parts  of  the  plant. 

Every  theory  of  heredity  has  to  take  into  account  this 
combination  of  the  hereditary  characters  into  larger  and 
smaller  groups,  and  different  authors,  like  Darwin  and 
Nageli  have  strongly  emphasized  this  point.  But  right 
here  lies  a  great  difficulty  which  interferes  with  a  working 
out  of  the  theory  in  detail,  for  in  many  cases  it  will  ob- 
viously be  extremely  difficult  to  decide  whether  one  is 
dealing  with  a  single  hereditary  character  or  with  a  small 
group  of  them.  There  is  here  a  large  field  for  morpho- 
logical analysis  which  awaits  working  up. 

§  5.   The  Combination  of  Hereditary  Characters 

Hereditary  characters  can  be  combined  to  any  extent 


Combination  of  Hereditary  Characters  25 

and  in  any  proportion.  This  is  shown  in  variegated  leaves 
and  striped  flowers,  where  the  result  of  this  combination, 
after  corresponding  splitting,  is  almost  directly  demon- 
strated to  us.  Almost  endless  is  the  diversity  of  pattern 
of  variegated  leaves,  frequently  on  the  same  plant,  or  at 
least  on  the  different  individuals  of  one  and  the  same 
crop.  Striped  flowers,  according  to  Vilmorin,  arise 
through  partial  atavism  from  old  white-flowering  varie- 
ties of  red  or  blue  species.10  Young  varieties  usually  re- 
vert by  leaps  to  the  ancestral  form,  while  the  older  ones 
do  so  by  steps,  through  the  appearance  of  isolated  stripes 
of  the  original  color  on  the  white  back-ground.  It  is  as 
if  the  color  potentialities  were  already  too  much  weakened 
to  tint  the  whole  corolla  in  one  effort.  The  descendents 
of  the  first  striped  flowers,  however,  soon  form  broader 
stripes,  and  finally  return,  after  a  few  generations,  [at 
least  in  some  specimens,11]  to  the  uniform  color  of  the  an- 
cestral form. 

Extremely  peculiar  are  those  cases  where  hereditary 
potentialities,  which  in  the  active  state  necessarily  ex- 
clude each  other,  occur  together  in  a  latent  state.  Instead 
of  giving  a  long  enumeration  of  many  cases,  I  prefer  to 
describe  a  well-known  case  of  variability,  and  select  for 
the  purpose  the  arrangement  of  leaves  in  whorls. 

Two-ranked  whorls,  the  leaves  of  which  stand  cross- 
wise over  each  other  on  the  successive  nodes,  belong  to 
the  best  and  most  constant  characteristics  of  entire  nat- 
ural families.  Less  frequent  are  the  cases  of  three-  and 
more-ranked  whorls.  Quite  frequently,  however,  one 

10Vilmorin,  L.  Leveque  de.  Notices  sur  V  ameliorations  des 
plantes  par  le  semis,  pp.  39-41.  1886.  (According  to  modern  views 
the  stripes  are  due  to  a  separate  character,  de  V.  1909.) 

"Matter  in  the  body  of  the  text  in  brackets  has  been  introduced 
anew  into  the  translation  by  the  author  of  the  original.  Tr. 


26    Mutual  Independence  of  Hereditary  Characters 

species  will  change  from  its  normal  type  into  another  form 
of  whorl,  and  in  numerous  plants  with  decussate  leaves, 
single  branches  with  three-  and  more-ranked  whorls 
have  been  observed.  The  Fuchsias  and  the  Weigelias  of 
our  gardens,  are  common  examples.  The  transitions  from 
one  number  in  the  whorls  to  the  other  usually  take  place 
by  leaps,  in  such  a  way  that  the  whole  shoot  springing 
from  one  bud  is  alike  in  this  respect;  however,  branches 
with  another  number  in  their  whorls  will  frequently  de- 
velop from  its  terminal  bud  or  its  lateral  buds.  More 
rarely  a  shoot  will  change,  during  its  development,  from 
one  number  to  another,  as  is  the  rule,  for  example,  in 
Lysimachia  vulgaris.  Intermediate  forms  between  two- 
or  three-  and  four-ranked  whorls  are  exceedingly  rare, 
although  from  our  present  knowledge,  they  may  develop 
quite  readily,  and  have  actually  been  observed  from  time 
to  time  in  most  plants  with  whorled  leaves.12  I  mean  those 
whorls  in  which  one  leaf  is  more  or  less  deeply  split  at  its 
apex,  while  the  mid-vein  forks.  This  splitting  occurs  in 
all  conceivable  degrees  and  leads  to  a  complete  doubling 
in  those  leaves  which  bear  two  blades  on  one  cleft  petiole. 
Consideration  of  numerous  examples  gives  the  impression 
that  the  single  whorl-forms  are  antagonistic  to  each  other, 
and  that  each  tries  to  exclude  the  other.  It  is  rare  that 
they  do  not  succeed  in  this  effort,  and  then  we  get  the 
above  mentioned  leaves  with  the  forked  mid-vein,  the 
complete  series  of  transition  of  which,  from  one  leaf  to 
two  leaves  has  been  figured  and  described  by  Delpino.13 

Therefore,  even  such  qualities,  which  in  the  devel- 
oped plant  exclude  each  other,  are  miscible,  apparently 

12Cf.    Delpino,   F.     Teoria   generale   della   Fillotassi.     Atti  R. 
Univ.  Genova  4:  197.    1883. 

™Loc.  cit.  p.  206,  Taf.  LX,  Fig.  60. 


Hereditary  Characters  Are  Units  27 

without  difficulty,  in  the  latent  state.  In  truth,  the  prin- 
ciple illustrated  by  this  example  holds  good  also  in  the 
phenomena  of  monoecism  and  dioecism,  of  the  di-  and 
trimorphism  of  flowers,  and  indeed,  throughout  the  en- 
tire range  of  organ-formation.  Everywhere  we  find 
characteristics  which  cannot  exist  simultaneously  in  the 
same  organ,  and  yet  must  be  associated  in  a  latent  state 
during  its  youth. 

In  summarizing  briefly  what  has  been  said,  we  see 
that  experiments  and  observations  on  the  origin  and  fix- 
ing of  variations  teach  us  to  recognize  hereditary  char- 
acters  as  units  with  which  we  can  experiment.  They 
teach  us  further  that  these  units  are  miscible  in  almost 
every  proportion,  most  experiments  really  amounting 
merely  to  a  change  in  this  proportion. 

The  above  considerations  are  verified  in  a  striking 
manner  by  experiments  in  hybridization  and  crossing.  In 
no  other  connection  does  the  concept  of  a  species  as  a 
unit  made  up  of  independent  factors  stand  forth  so 
clearly.  Everyone  knows  that  the  hereditary  characters 
of  two  parents  may  be  mixed  in  a  hybrid.  And  the  ex- 
cellent experiments  of  many  investigators  have  taught  us 
how,  in  the  descendents  of  hybrids,  an  almost  endless 
variation  can  usually  be  observed,  which  is  essentially  due 
to  a  mixing  of  the  characteristics  of  the  parents  in  a  most 
varied  manner. 

The  hybrids  of  the  first  generation  have  quite  definite 
characteristics  for  each  pair  of  species.  If  one  produces 
a  hybrid  of  two  species,  which  previous  investigators  have 
already  succeeded  in  crossing,  he  can,  as  a  rule,  rely  on 
the  description  given  of  it  tallying  exactly  with  the  newly 
produced  intermediate  form.  If  the  hybrid  is  fertile 
without  the  help  of  its  parents,  and  if  its  progeny  are 


28    Mutual  Independence  of  Hereditary  Characters 

grown  through  a  few  generations  in  thousands  of  speci- 
mens, one  can  almost  always  observe  that  hardly  any  two 
are  alike.  Some  revert  to  the  form  of  the  pollen-parent, 
others  to  that  of  the  pistil-parent ;  a  third  group  occupies 
a  central  position.  Between  these  are  placed  the  others 
in  the  most  motley  variety  of  staminate  and  pistillate 
characteristics  and  in  almost  every  gradation  of  mutual 
inter-mixture. 

Many  and  prominent  authors  have  pointed  out  the 
significance  of  hybrids  for  establishing  the  nature  of  fer- 
tilization. With  the  same  right  we  may  use  them  in  try- 
ing to  penetrate  into  the  mystery  of  specific  character. 
And  then  they  clearly  prove  to  us  that  this  character  is 
fundamentally  not  an  indivisible  entity.  The  character- 
istics of  a  hybrid  (of  the  first  generation)  are  as  sharply 
defined  and  as  constant,  and  on  the  whole  of  the  same 
order  as  those  of  the  pure  species,  and  the  frequent  spe- 
cific name,  hybridus,™  might  go  to  prove  that  even  the 
best  systematists  felt  this  agreement. 
•N  Kolreuter,  Gartner,  and  others  have  combined  in  one 
hybrid  two,  three,  and  more  species,  and  there  is  no  rea- 
son why  any  other  than  a  purely  practical  limit  should 
be  put  to  this  number,  and  that,  in  fact,  there  should  not 
be  combined  in  one  hybrid  characteristics  which  have 
been  taken  from  an  unlimited  series  of  allied  species. 
But  this  is  of  small  importance,  the  chief  point  being  the 
proposition  that  the  character  of  a  pure  species  like  that 
of  hybrids,  is  of  a  compound  nature. 

Crossings  of  varieties  of  the  same  species  belong,  es- 
pecially in  horticultural  practice,  to  the  most  common 
operations.  Ordinarily  the  object  pursued  is  simply  that 
of  producing  intermediate  forms.  Not  infrequently, 

14E.  g.  Papaver  hybridum  L.,  Trifolium  hybridum  L. 


Cross-  and  Self -Fertilization  29 

however,  one  desires  to  impart  single  definite  qualities  to 
one  variety  and  he  derives  these  from  another  variety, 
sometimes  even  from  another  species.  Hardening  against 
winter-frost  has  frequently  been  transmitted  in  this  man- 
ner from  one  form  to  another.  Carriere15  cites  instances  of 
Begonias  which,  through  crossing  with  a  variety  of  an- 
other species  with  variegated  leaves,  have  been  made 
varigated  without  having  their  other  qualities  changed. 
The  conviction  is  really  quite  general  in  horticultural 
practice  that,  by  crossings,  one  may  combine  the  charac- 
ters of  varieties  at  will,  and  thus  improve  his  races  ac- 
cording to  his  needs  in  many  as  well  as  in  individual 
desirable  points. 

§  6.  Cross-  and  Self-fertilisation 
In  addition  to  the  arguments  dealt  with  in  the  pre- 
ceding paragraph,  which  gives  us  the  results  of  ex- 
periments in  crossing  and  hybridization,  we  will  now 
consider  normal  fertilization  and  see  to  what  extent,  in 
this  domain,  the  facts  support  our  conception  of  the  mu- 
tual independence  and  miscibility  of  hereditary  charac- 
ters. 

To  fathom  the  meaning  of  fertilization  is  one  of  the 
most  difficult  problems  of  biology.  The  numerous  adapta- 
tions of  this  process  to  the  most  varied  conditions  of  life, 
and  the  powerful  influence  which  it  has  exercised  on  the 
differentiation  of  species,  especially  through  the  develop- 
ment of  the  secondary  sexual  characters,  threaten  always 
to  mislead  us,  and  to  make  us  mistake  its  essential  nature 
through  its  later  acquired  significance.  Here,  as  in  so 
many  cases,  the  conditions  in  the  plant  kingdom  are  clearer 

15Carriere,  E.  A.  Production  et  fixation  des  varietes,  p.  22.  1865. 
Other  examples  are  given  by  Verlot,  Sur  la  production  et  la  fixation 
des  varietes.  pp.  46  and  65.  1865.  Cf.  also  Darwin,  loc.  cit.  2:  73. 


30    Mutual  Independence  of  Hereditary  Characters 

and  simpler  than  in  the  animal  kingdom,  in  which  es- 
pecially the  exclusive  limitation  of  propagation  of  the 
higher  animals  to  the  sexual  method  makes  us  only  too 
easily  over-estimate  the  significance  of  this  process.  To 
this  must  be  added  the  fact  that,  for  the  vegetable  king- 
dom, quite  an  unexpected  light  has  been  thrown  on  the 
nature  of  this  process  through  the  exhaustive  compara- 
tive study  of  the  significance  of  cross-  and  self-fertiliza- 
tion, for  which  we  are  indebted  to  Darwin. 

Darwin's  experiments  have  taught  us  that  the  essence 
of  fertilization  consists  in  the  mixing  of  the  hereditary 
characters  of  two  different  individuals.16  Self-fertiliza- 
tion, which  takes  place  so  readily  in  the  vegetable  king- 
dom, and  is  so  easily  accomplished  experimentally,  has 
not  by  any  means  the  same  significance.  From  seeds 
obtained  in  the  last  named  manner  the  individuals  pro- 
duced were  always  weaker  in  Darwin's  experiments  than 
those  obtained  in  a  crop  from  crossed  flowers.  The 
first  ones  were  smaller,  with  less  profuse  branching,  flow- 
ering less  abundantly  and  less  constantly,  and  accordingly 
they  bore  less  seed.  Crossing  two  flowers  of  the  same 
plant  was  more  deterimental  than  the  pollination  of  the 
flowers  with  their  own  pollen. 

Even  the  crossing  of  different  individuals  was  not  suf- 
ficient to  keep  the  species  normal  when  it  was  cultivated 
year  after  year  in  the  same  bed,  and  protected  from  being 
fertilized  by  specimens  of  a  different  origin.  The  whole 
colony  deteriorated  steadily  and  distinctly  in  the  course 
of  a  few  years ;  not  only  did  the  plants  become  smaller  and 
weaker,  but  their  individual  differences  decreased  so  much 
that  they  resembled  each  other  almost  completely.  A 

16Darwin,  Origin  of  Species.  6  Ed.,  pp.  76-79,  and  Cross  and 
Self  Fertilisation  in  the  Vegetable  Kingdom.  1876. 


The  Essence  of  Fertilisation  31 

single  cross,  however,  of  such  a  colony  with  individuals 
of  another  origin  restored  the  original  vigor. 

The  process  of  fertilization,  in  its  essence,  does  not 
consist,  therefore,  in  the  union  of  two  sexes,  but  in  the 
mixing  of  the  hereditary  characters  of  two  individuals  of 
different  origin,  or  at  least  of  such  as  have  been  subjected 
to  different  external  conditions.  Therefore,  a  difference 
in  hereditary  characters  is  obviously  a  condition  for  at- 
taining the  full  advantage  of  fertilization ;  this  difference, 
however,  must  have  been  acquired  in  the  last  instance 
through  a  life  under  different  influences. 

Let  us  regard  the  individual  hereditary  factors  as  in- 
dependent units,  which  can  be  combined  with  each  other 
in  different  proportions  into  the  individual  character  of  a 
plant.  Let  us  further  assume  that  their  relative  increase 
or  decrease  depends  on  external  influences.  Evidently 
there  is  then  a  great  probability  that,  under  similar  ex- 
ternal conditions,  the  same  factors  will  deteriorate  in 
different  individuals,  while  under  different  conditions  this 
fate  will  befall  other  factors  in  every  individual.  Thus, 
on  crossing  the  plants  of  the  same  bed  only,  the  individual 
deviations  of  the  same  kind  are  strengthened;  the  weak- 
ened factors  are  therefore  made  still  weaker.  But  if  we 
cross  individuals  of  the  most  different  culture  possible, 
the  differences  in  the  individual  factors  are  clearly  bal- 
anced, at  least  in  part;  and  this  the  more  so,  the  more 
numerous  the  specimens  which  deviate  from  each  other, 
and  which  are  used  for  the  crossing. 

It  is  well  known  to  plant  breeders  that  luxurious  con- 
ditions which  are  varied  as  much  as  possible  lead  to  an 
accumulation  and  increase  of  individual  differences,  while 
simple  and  uniform  circumstances  make  them  disappear 
gradually,  and  thus  further  the  uniformity  of  all  speci- 


32    Mutual  Independence  of  Hereditary  Characters 

mens.  The  first  method  is  applied  in  improving  races,  the 
latter  in  fixing  newly  acquired  varieties. 

To  maintain  a  species  with  the  required  proportion  of 
all  its  hereditary  factors,  only  an  occasional  crossing  is 
necessary.  It  need  not  precede  every  generation.  Where 
sexual  generations  alternate  with  asexual  ones,  as  in  the 
gall-fly,  and  even  where  the  latter  occur  in  the  majority, 
as  in  many  aphids,  this  is  clearly  seen. 

With  bees  the  fertilized  eggs  become  females,  the  un- 
fertilized ones  males.  But  since  every  male  descends 
necessarily  from  a  female  that  originated  through  fer- 
tilization, it  evidently  profits  sufficiently  by  the  advant- 
ages of  an  occasional  crossing.  The  aphids,  in  which  the 
male  as  well  as  the  female  originate  parthenogenetically, 
teach  us  that  here  we  have  to  do  not  with  fundamental 
relations,  but  with  special  adaptations. 

The  never-opening,  so-called  cleistogamous  flowers, 
the  numerous  devices  for  insuring  self-fertilization  in 
flowers  in  case  they  are  not  visited  by  insects,  and  the 
almost  unlimited  use  of  vegetative  multiplication  in  plants, 
all  serve  to  teach  us  that  an  occasional  fertilization  is  all 
that  is  necessary  for  the  normal  preservation  of  the  spe- 
cies. That  in  higher  animals  every  individual  originates 
in  the  sexual  way,  is  therefore  obviously  only  a  special 
adaptation. 

In  summarizing  the  result  of  these  considerations,  we 
may  say  that  the  true  essence  of  fertilization  consists  in 
V mixing  the  hereditary  characters  of  the  different  individ- 
uals of  a  species.  Hybrids  have  taught  us  how  we  are  to 
conceive  this  co-mingling.  There  is  no  doubt  that  the  pro- 
cess of  mixing  is,  in  principle,  the  same  in  both  cases. 
And  just  as  Wichura17  succeeded  in  producing  hybrids 

17Wichura,  Max.  Bastardbefruchtung  im  Pflanzenreich  er- 
lautert  an  den  Bastarden  der  Weiden.  Breslau,  1865. 


Conclusion  33 

from  six  different  kinds  of  willows,  so  should  it  be  pos- 
sible to  combine,  by  crossing,  the  hereditary  qualities  of 
several  individuals  into  one. 

In  the  preceding  paragraphs  we  have  seen  how  the 
single  hereditary  characters  occur  as  independent  units 
in  the  experiments  of  hybridization  and  crossing,  and  how 
they  can  be  attained  in  almost  every  degree.  In  the  same 
way,  evidently,  must  we  think  of  those  units  as  inde- 
pendent in  the  ordinary  process  of  fertilization  as  well. 

§  7.   Conclusion 

Seemingly  elementary,  the  specific  character  is  ac- 
tually an  exceedingly  complex  whole.  It  is  built  of  nu- 
merous individual  factors,  the  hereditary  characters.  The 
more  highly  differentiated  the  species,  the  higher  is  the 
number  of  the  component  units.  By  far  the  most  of  these 
units  recur  in  numerous,  many  of  them  in  numberless  or- 
ganisms, and  in  allied  species  the  common  part  of  the 
character  is  built  up  of  the  same  units. 

On  trying  to  analyze  species  into  these  individual 
factors,  we  are  confused  by  their  number,  which,  in  the 
higher  plants  and  animals  reaches  probably  into  the 
thousands.  If,  however,  we  regard  the  entire  world  of 
organisms  as  the  subject  of  our  analysis,  then  the  total  ^ 
number  of  hereditary  characters  which  is  needed  for  the 
building  up  of  all  living  beings,  is  indeed  large  in  itself, 
but,  in  relation  to  the  number  of  species  it  is  small.  In 
that  limited  sphere  our  method  of  investigation  leads  ap- 
parently only  to  complications,  but,  on  the  whole,  it  evi- 
dently leads  the  way  towards  a  very  considerable  simpli- 
fication of  the  problems  of  heredity. 

The  hereditary  factors,  of  which  the  hereditary  charac- 
ters are  the  visible  signs,  are  independent  units  which  may 


34    Mutual  Independence  of  Hereditary  Characters 

have  originated  separately  as  to  time,  and  can  also  be  lost 
independently  from  one  another.  They  can  be  combined 
with  each  other  in  almost  every  proportion,  every  indi- 
vidual character  from  complete  absence  through  all 
gradations  being  capable  of  attaining  the  highest  devel- 
opment. Frequently  the  conditions  are  so  unfavorable 
for  some  of  them  that  they  cannot  manifest  themselves 
at  all,  and  so  remain  latent.  In  this  condition,  they  may 
either  persist  for  thousands  of  generations,  or  they  may 
appear  in  every  generation  during  the  development  of  the 
individual  from  the  fertilized  egg,  in  which  they  are  nearl) 
all  latent. 

The  hereditary  factors  compose  the  entire  specific 
character ;  there  is  no  separate  basis  to  which  they  are  at- 
tached. 

Although  independent  to  the  degree  that  each,  of 
itself,  can  become  weaker  and  even  disappear  completely, 
they  are  yet,  as  a  rule,  united  into  smaller  and  larger 
groups.  And  the  condition  is  such  that,  when  external 
influences,  such  as  a  stimulus  to  gall-formation,  bring  a 
definite  character  into  dominance,  the  entire  group  to 
which  it  belongs  is  usually  set  into  increased  activity. 

Independence  and  miscibility  are  therefore  the  most 
essential  attributes  of  the  hereditary  factors  of  all  or- 
ganisms. 

To  find  a  hypothesis  which  will  make  these  charac- 
teristics more  comprehensible  to  us,  is,  according  to  my 
opinion,  the  chief  problem  of  every  theory  of  heredity. 


B.    PREVAILING  VIEWS  ON  THE  BEARERS  OF  HEREDITARY 
CHARACTERS 


CHAPTER  II 

THE  SIGNIFICANCE  OF  THE  CHEMICAL  MOLECULES  OF 

THE  PROTOPLASM  WITH  REFERENCE  TO  THE 

THEORY  OF  HEREDITY 

§  i.  Introduction 

According  to  our  present  conception  of  all  nature,  the 
wonderful  phenomena  of  heredity  must  have  a  material 
basis,  and  this  basis  can  be  no  other  than  the  living  pro- 
toplasm. Every  cell  originates  through  the  division  of 
one  that  already  exists;  the  living  substance  of  the 
mother-cell  is  distributed  among  the  individual  daughter- 
cells  and  passes  into  them  with  all  its  hereditary  qualities. 
Microscopic  investigation  of  the  cell-body  and  the  art  of 
the  breeder,  so  far  apart  from  each  other  until  recently, 
come  nearer  and  nearer  to  working  hand  in  hand.  And  it 
is  only  through  the  co-operation  of  these  two  great  lines 
of  human  thought  that  we  can  succeed  in  establishing  the 
basis  for  a  theory  of  heredity. 

Chemistry  teaches  us  that  living  protoplasm,  like  any 
other^substance,  must  be  built  up  of  chemical  molecules, 
and  that  a  final  explanation  of  the  phenomena  of  life  can 
be  reached  only  when  we  shall  succeed  in  deriving  the 
processes  in  protoplasm  from  the  grouping  of  its  mole- 
cules, and  from  the  composition  of  the  latter  out  of  their 
atoms. 

We  are  still,  however,  very  far  from  this  goal.  The 
chemists  study  chiefly  pure  bodies,  that  is,  such  as  are 
built  up  from  like  molecules;  but  protoplasm  is  evidently 
a  mixture  of  numerous,  if  not  of  almost  countless  differ- 
ent chemical  compounds.  And  by  far  the  most  of  these 


38  The  Significance  of  Chemical  Molecules 

latter  have  been,  even  chemically,  only  very  incompletely 
investigated. 

Of  course,  this  consideration  must  not  keep  us  from 
utilizing  the  great  truths  of  chemistry  in  the  explanation 
of  life  processes.  Haeckel,  and  many  other  investigators 
after  him,  have  pointed  out  the  great  significance,  for 
such  an  explanation,  of  the  power  of  carbon  to  combine 
^in  the  most  varied  relations  with  other  elements.  "This, 
in  its  way,  unique  property  of  carbon  we  must  designate 
as  the  basis  of  all  pecularities  of  the  so-called  organic 
compounds."1  The  differences,  which  occur  in  the 
growth  of  organic  and  inorganic  individuals,  are  due  to 
the  more  complex  chemical  composition  and  the  power 
of  imbibition  of  many  carbon-compounds,2  et  cetera. 

In  chemistry  also  this  importance  of  carbon  has  been 
\  emphasized.  In  his  Vieivs  on  Organic  Chemistry,  van't 
Hoff3  says :  "From  the  chemical  properties  of  carbon 
it  appears  that  this  element  is  able,  with  the  help  of  two 
or  three  others,  to  form  the  numberless  bodies  which  are 
necessary  for  the  manifold  needs  of  a  living  being;  from 
their  almost  equal  tendency  to  combine  with  hydrogen 
and  oxygen,  follows  the  capacity  of  the  carbon-com- 
pounds to  be  adapted  alternately  for  processes  of  r^duc- 
tion  and  of  oxydation  as  the  simultaneous  existence  of  a 
vegetable  and  an  animal  kingdom  requires."  And,  after 
a  discussion  of  the  influence  of  temperature  on  the  change 
of  the  chemical  property  of  carbon,  he  continues  :  "There- 
fore, one  does  not  go  too  far  in  assuming  that  the  ex- 
istence of  the  vegetable  and  animal  world  is  the  enor- 

iHaeckel,  E.   Generelle  Morpholgie,    1:    121.    Berlin.    1886. 
2Loc.  cit.  p.  166,  and  Haeckel,  E.    Die  Perigenesis  der  Plastidule. 
p.  34.    1876. 

3 Van't  Hoff.  Ansichten  iiber  die  organischc  Chcmic.  1:  26.    1878. 


Tivo  Kinds  of  Life-Processes  39 

mous  expression  of  the  chemical  properties  which  the 
carbon-atom  has  at  the  temperature  of  our  earth." 

Furthermore  if  we  take  into  consideration  the  num- 
berless isomers,  which  especially  the  more  complicated 
compounds  of  carbon,  such  as  protein  bodies,  can  form, 
according  to  the  present  chemical  theories,  there  can 
hardly  be  any  doubt  that  we  shall  some  day  succeed  in  re- 
ducing the  hereditary  characters  of  all  organisms  to  chem- 
ical differences  of  their  protoplasmic  basis.4 

But,  much  as  such  general  considerations  may  help  to 
further  our  need  for  a  uniform  conception  of  all  nature, 
they  are  still  far  from  serving  us,  especially  at  the  present 
time,  as  a  basis  for  a  theory  of  heredity. 

Experimental  physiology  of  plants  and  animals  has 
succeeded  in  reducing  many  of  the  processes  of  life  to  the 
chemical  effects  of  the  involved  compounds,  to  repeat 
them  in  part  outside  of  the  organism,  but  in  part  also  to 
demonstrate  the  fact  that  their  behavior  in  the  living  body 
is  ruled  by  the  general  laws  of  chemistry.  Into  an 
understanding  of  the  processes  of  breathing,  nutrition, 
and  metabolism  we  have  been  initiated  in  a  simply  as- 
tonishing manner  by  numerous  investigators,  and  the 
purely  mechanical  manifestations  of  energy  which  ac- 
company growth  and  motion  have  also,  in  great  part, 
been  analyzed  and  reduced  to  general  laws.  But  the  chief 
discovery  of  these  studies  is  that  two  kinds  of  processes 
occur  in  the  living  body.  In  the  first  place,  those  that  are 
separable  from  living  substance,  and  can  therefore  be  ar- 
tificially imitated,  or  even  exactly  duplicated.  In  the 
second  place,  those  that  are  inseparable  from  that  sub- 
stratum, and  which  indeed  find  their  existence  in  the 

4Cf.  Haeckel,  E.  Generelle  Morphohgie.  1:  277,  and  Sagiura, 
Shigetake.  Nature  27:  103.  1882. 


40          The  Significance  of  Chemical  Molecules 

processes  of  life  of  that  very  substratum.  The  former 
processes  are  purely  physical  or  chemical ;  in  a  word,  they 
are  aplasmatic  processes ;  the  latter  ones  we  must  designate 
as  plasmatic;  that  is,  as  taking  place  in  the  molecules  of 
the  living  protoplasm  itself.  The  former  belong  to  phy- 
siological chemistry  and  physics,  the  latter  form  the 
proper  subject  of  physiology.  But  toward  an  under- 
standing of  the  latter  we  have  taken  only  the  first  steps. 

It  is  neither  by  general  considerations,  nor  on  an  ex- 
perimental basis,  that  we  can  penetrate,  at  the  present 
moment,  into  the  relations  between  the  qualities  of  the 
chemical  molecules  of  the  protoplasm  and  the  phenomena 
of  heredity.  It  can  therefore  be  only  a  matter  of  try- 
ing, by  means  of  hypotheses,  to  get  an  insight  into  these 
relations. 

It  is  evident  that  we  are  justified  in  making  such  an 
attempt.  This  right  is  very  generally  acknowledged,  for 
several  prominent  investigators  have  published  their 
views  on  this  subject.  Some  have  even  made  their  hy- 
potheses accessible  to  the  critical  consideration  of  others 
by  working  out  logically  the  consequences  arising  there- 
from. And  certainly,  no  one  can  doubt  for  a  moment  that 
these  hypotheses,  much  as  they  differ  at  present,  have 
aroused  scientific  interest  in  these  questions. 

The  directions  which  these  hypotheses  take  can,  I  be- 
lieve, be  summarized  under  three  heads.  Some  authors 
go  directly  back  to  the  chemical  composition  of  proto- 
plasm and  seek  to  derive  the  life-processes  from  it. 
Others  again  assume  that  the  chemical  molecules  are  com- 
bined into  larger,  but  still  invisibly  small  organic  units, 
and  regard  these  units  as  the  real  bearers  of  heredity. 
Some  of  them  imagine  that  these  units  always  represent 
the  whole  specific  character,  and  that  therefore  the  in- 


Protoplasm  and  Protein  41 

dividual  bearers  of  heredity  in  the  same  cell,  with  the 
exception  of  insignificant  differences,  are  alike.  •  Finally, 
there  is  the  directly  opposite  opinion  of  those  investi- 
gators who  assume  a  special  kind  of  material  bearer  for 
every  individual  hereditary  character;  and  according  to 
whom,  therefore,  protoplasm  is  built  up  of  numberless 
unlike  hypothetical  units. 

It  is  these  three  different  principles  that  we  will  sub- 
ject to  a  thorough  comparative  examination  in  this  and 
the  two  following  chapters.  Before  doing  so,  however, 
we  must  first  critically  consider  the  relation  between  pro- 
tein substances  and  protoplasm. 

§  2.  Protoplasm  and  Protein 

Lately  the  conceptions  of  protoplasm  and  protein  have 
been  confused  by  many  authors.5  This  has  led  to  the 
hypothetical,  and  in  no  way  justified  assumption  of  a 
living  protein.6  This  usage  has  exercised  its  influence, 
even  on  the  theory  of  heredity,  and  for  this  reason  it 
should  not  remain  unmentioned  here.  Without  this  con- 
fusion, the  view  which  regards  the  chemical  molecule  of 
protoplasm  as  the  bearer  of  the  hereditary  characters 
would  probably  never  have  met  with  any  favor. 

Protein  is  a  chemical,  protoplasm  a  morphological 
concept.  Chemistry  is  able  to  produce  many  pure  pro- 
teins, while  the  nature  of  protoplasm  is  conditioned  by 
its  very  heterogenous  composition.  Many  protein  bodies 
can  pass  into  solution,  but  nobody  will  ever  think  it  pos- 
sible to  obtain  a  solution  of  protoplasm  in  a  test-tube. 

5Haeckel  refers  to  protoplasm  as  a  protein  body:  Generelle 
Morphologic.  1:  278. 

8A  term  proposed  by  Pfluger.  Arch.  Ges.  Physiol  10:  251.  1875. 
Tr. 


42          The  Significance  of  Chemical  Molecules 

Protein  bodies  are  indeed  products  of  life,  but  not  the 
bearers  thereof;  they  do  not  offer  us,  in  the  chemical 
laboratory,  any  essentially  different  quantities  than  the 
other  more  complicated  compounds.  Protoplasm,  how- 
ever, is  the  bearer  of  life;  it  is  distinguished  from  all 
chemical  substances  by  its  power  of  assimilation  and  of 
reproduction.  The  nature  of  these  two  processes  will 
undoubtedly  be  recognized  some  day,  but  up  to  the  pres- 
ent time  they  are  still  in  complete  darkness,  and  even 
the  boldest  minds  have  not  yet  succeeded  in  lifting  even 
as  much  as  a  corner  of  the  veil  that  covers  them. 

The  designation  of  protoplasm  as  a  protein  body,  or 
as  a  mixture  of  such  bodies,  is  based  upon  chemical  analy- 
ses and  micro-chemical  reactions.  The  latter  undoubt- 
edly betray  the  quite  common  presence  of  protein  in  pro- 
toplasm. But  the  explanation  of  this  fact  is  obvious. 
Protein  can  very  well  be  dissolved  in  the  water  of  imbi- 
bition of  protoplasm,  since  it  can  be  proven  to  occur  fre- 
quently in  solution  in  the  cell-sap.  It  is  even  not 
improbable  that,  in  killing  the  protoplasts,  protein  bodies 
are  frequently  formed.  But,  in  order  to  be  able  to  assert 
that  protoplasm  and  protein  are  identical,  it  ought  at  least 
to  be  demonstrated  that  protein-reactions  are  lacking 
neither  in  any  protoplasm  nor  in  any  individual  organ 
thereof.  But  such  does  not,  by  any  means,  appear  to  be 
the  case.7  Nucleus,  trophoplast,  and  nucleo-plasm,  have, 
it  is  true,  never  been  observed  without  protein,  in  well 
nourished  cells ;  but,  whether  the  wall  of  the  vacuoles  and 
the  plasma-membrane  are  structures  that  contain  protein, 
is  still  very  questionable.8 

Chemical  analyses  have,  without   doubt,   brought  to 

7Cf.  Zacharias,  E.  Bot.  Ze'it.  4:  209.    1883. 
8Cf.  Jahrb.  Wiss.  Bot.  14:  512.    1883. 


Morphological  Units  43 

light  important  conclusions  concerning  many  compounds 
developed  from  protoplasm.  But  whether  those  com- 
pounds were  present,  as  such,  in  the  living  protoplasm,  or 
have  only  developed  after  death,  or  through  the  influence 
of  reagents,  as  products  of  decomposition,  is  another 
question. 

The  chief  point  for  the  theory  of  heredity  is,  however, 
that  protoplasm  always  offers  us  certain  historical  char- 
acters besides  physical  and  chemical  properties.  It  is  to 
these  that  it  owes  its  peculiarity.  A  synthetic  composition 
of  protein  bodies  is  no  longer  regarded  by  anybody  as  an 
impossibility;  but  whether  we  shall  ever  succeed  in  ob- 
taining living  protoplasm  in  any  other  than  the  phyloge- 
netic  way,  will  probably  remain  for  a  long  time  a  matter  of 
well-founded  doubt. 

The  historical  characters  demand  a  molecular  struc- 
ture of  such  complicated  nature  that  the  chemistry  of  the 
present  time  fails  us  entirely  in  our  attempts  at  an  ex- 
planation. For  the  present,  therefore,  theory  must  be^ 
content  to  accept  the  idea  that  protoplasm  is  composed  of 
morphological  units.  These,  of  course,  must  themselves 
be  built  up  from  chemical  molecules,  and  among  the  latter 
the  protein  bodies  must  play  an  important  role.  To  con- 
clude from  this  fact,  however,  that  protoplasm  itself  is  a 
protein  body,  seems  not  at  all  justified. 

Those  invisible  morphological  units  are  of  a  hypothet- 
ical nature  and  we  will  not  follow  up  this  subject  any 
further  in  this  connection.  I  only  wished  to  show  how 
this  consideration  also,  leads  us  to  that  assumption  of 
pangens,  with  which  we  shall  have  to  deal  in  the  last  two 
chapters  of  this  section. 


44          The  Significance  of  Chemical  Molecules 

§  j.  Elsberg's  Plastidules 

The  most  thorough  attempts  to  explain  the  phenomena 
of  heredity  by  the  qualities  of  the  molecules  of  living 
matter  were  made  by  Louis  Elsberg  and  Ernst  Haeckel. 
Elsberg,  who  called  the  cells  plastids,  chose  for  the  com- 
ponent particles  the  name  of  plastid-molecule  or,  abbre- 
viated, plastidule.9  Haeckel  considered  this  expression 
a  brief  and  suitable  designation  for  the  polysyllable  pro- 
toplasm-molecule,10 and  secured  general  consideration  for 
the  term  in  his  "Perigenesis  of  the  Plastidule."11 

According  to  Elsberg,  living  matter  consists  entirely 
of  plastidules  which  multiply  in  such  a  manner,  through 
nutrition,  assimilation,  and  growth,  that  new  molecules 
with  the  same  characters  as  those  present,  are  constantly 
developed.  At  each  cell-division  these  are  transmitted  to 
the  daughter-cells.  The  resemblance  of  children  to 
their  parents,  grand-parents,  and  ancestors  is  explained 
in  a  simple  manner  by  saying  that  they  are  essentially 
built  up  of  the  same  kind  of  plastidules,  which  they  have 
inherited  from  their  ancestors.  All  individuals  of  one 
species  consist,  on  the  whole,  and  apart  from  incidental 
varieties,  of  the  same  plastidules;  every  species,  how- 
ever, contains  the  plastidules  of  its  whole  ancestry,  and 
consists  therefore,  of  as  many  different  plastidules  as 
there  were  different  species  in  this  ancestry.  The  dif- 
ferences between  individual  species  are  conferred  by  their 

9Elsberg,  Louis.  Regeneration,  or  The  Preservation  of  Organic 
Molecules :  a  Contribution  to  the  Doctrine  of  Evolution.  Proc. 
Amer.  Assoc.  Adv.  Sci.  23:  1874;  and  Elsberg,  Louis.  On  the  Plasti- 
dule Hypothesis.  Ibid.  Buffalo  Meeting,  August,  1876.  25:  178.  1877. 

10Haeckel,  E.  Jenaische  Zeits.  Med.  Naturw.  7:  536.   1873. 

"Haeckel,  E.  Die  Peregenesis  der  Plastidule.  p.  35.  Berlin,  1876. 


Elsberg's  Plastidules  45 

descent,  and  are,  therefore,  materially  based  on  the  dif- 
ferences of  the  plastidules.  Systematic  affinity  depends 
upon  the  possession  of  the  same  plastidules,  systematic 
differences  on  the  presence  of  different  molecules  in  addi- 
tion to  the  bulk  of  those  that  are  alike. 

Haeckel,  who,  in  his  "Generelle  Morphologic,"  had 
not  yet  considered  the  significance  of  the  molecule  for  the 
theory  of  heredity,12  has  further  carried  out  Elsberg's 
train  of  thought13  in  his  above  mentioned  monograph. 
"The  sum  total  of  physical  and  chemical  processes,  called 
life,  is  evidently  conditioned  in  the  last  instance  by  the 
molecular  structure  of  the  plasson."1*  In  the  non-nu- 
cleated plasson  (or  protoplast)  the  plastidules  are  every- 
where uniform;  in  the. nucleated  ones  they  are  differen- 
tiated in  such  a  manner  that  a  distinction  must  be  made 
between  plasmodules  and  coccodules  (nucleo-molecules). 
The  differentiation  of  the  organism  into  organs,  and  the 
division  of  labor  thereby  achieved,  Haeckel  attributes  to 
a  division  of  labor  of  the  plastidules,  for  in  this  way  they 
are  segregated  more  or  less,  and  thus  produce  the  various 
kinds  of  protoplasm.  Fertilization  consists  in  the  fusion 
of  two  protoplasts  which  have  developed  in  different 
directions  through  a  far-reaching  differentiation  of  their 
plastidules.15 

We  will  limit  ourselves  to  this  part  of  the  theory  of 

12Only  in  a  general  way  does  Haeckel  point  here  to  the  signifi- 
cance of  "the  numerous  and  minute  differences  in  the  atomic  con- 
stitution of  the  protein-compounds,  which  form  the  plasma  of  the 
plastids."  Gen.  Morphol  1:  277. 

13Elsberg  later  (Proc.  Amer.  Assoc.  Adv.  Sci.  25:  178.  1877.)  in- 
sisted that  he  had  been  misunderstood  and  misinterpreted  by  Haeckel 
in  the  monograph  above  referred  to.  Tr. 

l4Perigenesis.  p.  34. 

15Loc.  cit.  p.  52. 


46          The  Significance  of  Chemical  Molecules 

the  plastidules,  and  not  enter  into  the  speculations  on  the 
undulating  motion  of  these  particules.  But,  in  critically 
discussing  that  part,  we  can  emphasize  here  the  fact  that 
the  theory  is  composed  of  two  hypotheses : 

1.  Protoplasm  is  made  up  of  numerous  small  units, 
which  are  the  bearers  of  the  hereditary  characters. 

2.  These  units  are  to  be  regarded  as  identical  with 
molecules. 

Trie  first  of  these  two  hypotheses  has  obviously  very 
great  advantages.  It  explains  the  fundamental  phenom- 
ena of  heredity  in  a  simple  manner,  and  especially  ac- 
counts sufficiently  for  the  independence  and  miscibility 
of  the  individual  hereditary  characters.  It  is  identical 
with  the  first  law  of  Darwin's  pangenesis,  as  we  shall  see 
more  in  detail  in  the  third  Chapter.  We  shall,  therefore, 
put  off  a  more  thorough  discussion,  especially  as  Elsberg 
wrote  a  few  years  later  than  Darwin,  and  in  not  nearly 
as  clear  a  manner. 

Let  us  now  turn  to  a  criticism  of  the  second  thesis. 
Elsberg  never  expresses  himself  clearly  about  the  identity 
of  his  plastidule  with  chemical  molecules.  He  defines 
them  as  the  smallest  particles  of  a  cell  in  which  the  hered- 
itary characters  lie  hidden.16  These  particles  must  be 
larger  than  the  molecules  of  the  ordinary  protein  bodies ; 
this  follows  from  their  much  more  complicated  character. 
Haeckel,  however,  devotes  a  detailed  discussion  to  this 
identity.17  "T*he  plastidules  possess,  first  of  all,  every 
quality  which  physics  ascribes  generally  to  the  hypotheti- 
cal molecules,  or  combined  atoms.  Consequently  each 
plastidule  cannot  be  analyzed  any  further  into  smaller 
plastidules,  but  only  into  its  component  atoms. ..." 

18Elsberg.  loc.  clt.  p.  9. 
17Perigenesis  loc.  cit.  pp.  35-36. 


Elsberg's  Plastidules  47 

As  long  as  we  are  concerned  only  with  the  explana- 
tions of  the  chemical  processes  in  cell-life,  this  hypothesis 
is  certainly  highly  satisfactory.  The  production  of  vari- 
ous compounds,  as  for  example,  the  red  coloring  matter 
of  a  flower,  can  be  imagined  as  a  function  of  definite 
molecules  of  the  protoplasm,  more  or  less  in  the  same 
manner  as  the  action  of  enzymes  or  chemical  ferments. 
Even  the  secretion  of  cellulose  one  might  try  to  explain 
thus  by  analogy.  As  soon,  however,  as  we  have  to  do 
with  morphological  processes,  this  hypothesis  fails  us  en- 
tirely, because  the  frequently  attempted  comparison  with 
the  formation  of  crystals  furnishes  only  a  remote  simi- 
larity. The  hypothesis  is  quite  useless  when  applied  to' 
that  peculiar  attribute  of  life,  growth  through  assimila- 
tion. It  is  obvious  that  any  attempt  to  explain  life-pro- 
cesses from  the  properties  of  chemical  molecules  must 
consider  this  phenomenon  first  of  all.  But  in  the  great 
realm  of  the  lifeless  there  is  no  analogy  for  it.  Chemical 
molecules  do  not  grow  in  such  a  way  as  to  separate  later 
into  two  molecules  which  are  like  the  original  one.  They 
do  not  assimilate,  and  in  this  sense  they  are  not  capable 
of  independent  multiplication.  They  do  not  possess  any 
qualities  at  all  from  which  one  could  at  present  hypotheti- 
cally  explain  a  growth  through  assimilation. 

Here  lies  the  great  difficulty  of  the  plastidule  hy- 
pothesis. Indeed,  Haeckel  says,  "Besides  the  general 
physical  properties,  which  modern  physics  and  chemistry 
ascribe  to  the  molecules  of  matter  in  general,  plastidules 
possess  some  special  attributes  which  are  exclusively 
their  own,  and  these  are,  quite  generally  speaking,  the 
life-attributes  which,  according  to  the  present  concep- 
tion, distinguish  the  living  from  the  dead,  the  organic 
from  the  inorganic."  But  it  is  easily  understood  that  by 


48          The  Significance  of  Chemical  Molecules 

such  an  ancillary  hypothesis  the  meaning  of  the  hypothesis 
as  a  whole  is  changed.  For,  with  the  same  right,  one 
might  say  that  the  plastidules  are  not  molecules  at  all,  in 
the  sense  of  physics,  but  are  distinguished  from  them 
by  their  very  life-properties. 

It  would  be  easy  further  to  criticise  the  plastidule- 
hypothesis  in  the  same  direction.  It  leads  to  pure  specu- 
lation. According  to  Haeckel,  we  must  attribute  sensa- 
tion and  will  power  to  atoms.18  The  plastidules  possess 
memory,  according  to  his  theory ;  this  faculty  is  lacking 
in  all  other  molecules.19  We  shall  not  discuss,  either,  the 
wave  motion  of  the  plastidule. 

What  is  of  interest  to  us,  is  to  show  that  any  attempt, 
at  the  present  time  to  reduce  life-phenomena  to  the  prop- 
erties of  the  molecules  of  living  matter,  is,  to  say  the 
least,  premature.  We  must  either  limit  ourselves,  with 
Elsberg,  to  such  deductions  as  can  be  derived  from  Dar- 
win's gemmule-hypothesis,  or  be  compelled  to  resort 
everywhere  to  ancillary  hypotheses,  in  place  of  explana- 
tions. If  we  choose  the  first  method,  we  arrive  naturally 
at  the  assumption  of  invisible  units,  of  a  higher  order 
than  the  molecules  of  chemistry,  and  of  such  a  compli- 
cated composition  that  every  one  of  them  must  be  made 
up  of  a  large  number  of  chemical  molecules.  To  these 
units  we  must  attribute  growth  and  multiplication  as 
qualities  which  so  far  cannot  be  explained.  In  a  like  in- 
explicable manner  we  must  further  assume  that  they  are 
the  material  substratum  for  hereditary  characters.  Leav- 
ing this  part  unexplained,  we  can  clear  up  many  other 
things.  But  in  that  case  we  cannot  revert  to  the  mole- 
cules of  protoplasm. 

"Haeckel  loc.  cit.  p.  38. 
™Loc.  cit.  p.  40. 


The  Name  Molecule  Inappropriate  49 

Therefore  the  material  bearers  of  hereditary  charac- 
ters cannot  be  identical  with  the  molecules  of  chemistry; 
they  must  be  conceived  of  as  units,  built  up  from  the  latter, 
much  larger  than  they,  and  yet  invisibly  small. 

It  does  not  seem  to  me  correct  to  apply  the  name  mole- 
cule, or  living  molecule,  to  these  units.  This  appellation 
must  lead  to  confusions  and  misunderstandings,  and 
I  suppose  it  is  employed  only  from  lack  of  a  simple  desig- 
nation. As  such  a  term,  the  name  "pangen,"  proposed  in 
the  Introduction  (p.  7),  may  be  adopted. 


CHAPTER  III 

THE   HYPOTHETICAL   BEARERS    OF    SPECIFIC 
CHARACTERS 

§  4.    Introduction 

The  majority  of  investigators  assume  that  the  ma- 
terial bearers  of  hereditary  characters  are  units,  each  of 
which  is  built  up  of  numerous  chemical  molecules,  and  is 
altogether  a  structure  of  another  order  than  the  latter. 

1  Growth  through  assimilation,  and  multiplication 
by  division  are  always  assumed  for  them.  For  this 
reason,  as  Darwin  has  already  said,  they  are  rather  to  be 
placed  in  a  class  with  the  smallest  known  organisms,  than 
with  the  real  molecules.  An  explanation  of  these  prop- 
erties is  not  attempted ;  they  are  simply  accepted  as  a  fact. 
Neither  does  the  theory  of  heredity  require  such  an  ex- 
planation; it  can,  for  the  time  being,  be  reserved  as  a 
problem  for  a  later  theory  of  life. 

A  second  assumption  in  regard  to  the  nature  of  those 
hypothetical  units  is  still  .needed ;  namely,  one  concerning 
their  relation  to  the  hereditary  characters.  As  to  the  man- 
ner in  which  the  latter  are  determined  by  the  structure  of 
the  bearers  no  suppositions  are  yet  made,  for  the  theory 
of  heredity  does  not,  for  the  present,  need  this  elabora- 
tion. The  only  question  is,  whether  the  units  are  the 
bearers  of,  all  the  specific  attributes,  or  of  the  individual 
hereditary  characters  only.  Spencer  and  Weismann  are 
the  chief  representatives  of  the  first  view,  Darwin's  pan- 
genesis  assumes  the  latter. 


Spencer's  Physiological  Units  .   '  51 

We  have  now  critically  to  compare  these  various 
opinions.  In  doing  so  the  chief  question  is  in  how  far 
the  hypotheses  themselves,  as  they  have  just  been  de- 
scribed, and  without  further  ancillary  hypotheses,  can 
lead  to  an  explanation  of  the  phenomena  of  heredity. 

§  5.    Spencer's  Physiological  Units 

In  his  famous  system  of  Synthetic  Philosophy,  Her- 
bert Spencer  attempted,  probably  for  the  first  time,  to 
formulate  a  material  conception  of  heredity.  His  Prin- 
ciples of  Biology,  which  form  the  second  and  third  volume 
of  that  system,  appeared  in  1864  and  1867,  therefore 
before  the  publication  of  Darwin's  pangenesis  (1868). 
His  train  of  thought  is  essentially  as  follows : 

Bud-formation  from  leaves,  et  cetera,  teaches  us  that 
the  living  particles  of  these  organs  possess  the  power  of 
reproduction,  which  is  also  shown  in  animals  by  the  res- 
toration of  lost  members.  Now  these  particles  cannot  be 
the  cells  themselves,  because  some  cells  can  also  replace  lost 
parts.  Just  as  little  can  they  be  chemical  molecules,  be- 
cause these  are  much  too  simply  constructed  for  an  ex- 
planation of  all  the  morphological  differences.  They 
must,  therefore,  be  units  of  intermediate  size,  invisibly 
small,  but  composed  of  numerous  molecules.  Spencer20 
calls  them  physiological  units. 

Every  one  of  these  units  represents  the  entire  specific 
character;  slight  dissimilarities  in  their  structure  cause 
the  differences  between  allied  species  (p.  183). 

Spencer  finds  it  difficult  to  explain  fertilization. 
There  is  no  sense  in  it  unless  there  is  some  kind  of  dif- 
ference between  the  two  groups  of  physiological  units. 

20Spencer,  H.  Principles  of  Biology.  Ed.  2.  1:    180-183. 


52       Hypothetical  Bearers  of  Specific  Characters 

This  makes  him  assume  that  the  units  of  different  indi- 
viduals are  slightly  dissimilar.  From  this  it  follows  that 
in  the  child  the  two  kinds  of  units  of  both  parents  are 
mixed,  in  the  grandchild  the  four  different  units  of  the 
grandparents,  and  so  on.  In  this  way  one  would  arrive 
at  just  the  opposite  of  what  was  at  first  assumed,  namely, 
the  similarity  of  all  units  in  the  same  individual  (pp.  253, 
254,  and  267). 

To  escape  this  difficulty  Spencer  points  to  hybrids.  In 
these  the  physiological  units  of  two  species  are  mixed. 
The  hybrids  are  liable  to  be  inconstant  in  the  following 
generations,  and  to  revert  to  the  parental  forms.  There- 
fore the  unlike  physiological  units  oppose  a  mixture,  they 
repulse  each  other,  and  try  each,  by  excluding  the  dis- 
similar kind,  to  form  the  whole  individual  (p.  268).  In 
the  same  manner  the  unlike  physiological  units  exclude 
each  other  in  normal  fertilization,  and  in  this  way  uni- 
formity within  the  individual  is  sufficiently  assured. 

The  physiological  units  multiply  at  the  expense  of  the 
nutrient  material  (p.  254)  and  thus  produce,  as  a  rule, 
new  units  that  are  quite  alike.  Under  the  influence  of 
external  circumstances,  however,  they  sometimes  undergo 
slight  changes  during  the  process  of  their  multiplication, 
and  this  is  the  cause'  of  their  variability  (p.  287). 
Through  fertilization,  however,  the  balance  thus  disturbed 
is  regained  (p.  289). 

On  this  basis  heredity  is  easily  explained ;  it  is  founded 
on  the  fact  that  the  child  receives  from  father  and  mother 
the  material  units  that  go  to  make  up  its  characters. 
Strong  resemblance  of  the  child  to  one  of  its  two  parents 
is  due  to  the  predominance  of  the  respective  physiological 
units;  atavism  depends  upon  the  presence  of  units  in- 
herited from  some  given  ancestor.  Many  other  phenom- 


Wcisni ami's  Ancestral  Plasms  53 

ena   are    explained    by    Spencer    in    a    similarly    simple 
manner. 

Spencer's  theory  has,  without  doubt,  the  advantages 
of  a  clear  and  concise  system.  But  it  does  not  take  into 
account  the  train  of  thought  developed  in  our  first  section. 
On  the  basis  of  those  general  considerations,  therefore, 
the  theory  is  insufficient.  Especially  can  it  not  explain 
in  a  satisfactory  manner  the  differentiation  of  organs,  and 
any  attempt  to  bring  it  into  accord  with  this  process 
would  prove  its  fundamental  inadequacy.  Since  the  same 
thing  is  likewise  true  of  Weismann's  theory  of  the  ances- 
tral plasms  I  refer  the  reader,  in  regard  to  it,  to  the  con- 
clusion of  the  next  Section. 

§  6.    Weismann's  Ancestral  Plasms 

In  a  series  of  thoughtful  writings  during  the  last 
decade,  August  Weismann  has  aroused  the  general  in- 
terest of  the  scientific  public  in  the  principles  of  heredity. 
In  doing  so,  he  used,  as  a  basis,  the  most  recent  achieve- 
ments in  the  domain  of  cell-theory  and  the  process  of 
fertilization. 

Proceeding  from  the  conviction  that  the  development 
of  children  from  material  particles  of  their  parents  is  the 
cause  of  heredity,  and  that  the  solution  of  the  great 
mystery  is,  in  truth,  to  be  looked  for  in  the  molecular 
structure  of  the  protoplasm,  he  tries  to  form  a  definite 
conception  of  this  structure.  He  begins  by  saying  that, 
in  lower  organisms,  which  do  not  possess  a  sexual  dif- 
ferentiation, the  germ-plasm  of  each  individual  must 
still  be  completely  uniform.  During  fertilization,  how- 
ever, a  mixing  of  the  two  parental  germ-plasms  must  take 
place,  and  thus  in  the  child  there  are  mixed  two,  in  the 


54      Hypothetical  Bearers  of  Specific  Characters 

grand-child  four  kinds  of  germ-plasms.21  In  the  children 
of  the  first  sexually  produced  generation  there  will  be  only 
one-half  of  the  original  amount  of  the  two  kinds  of  germ- 
plasm,  in  the  grand-children  only  one  quarter.  In  every 
succeeding  generation  the  germ-plasm  will  consequently 
consist  of  a  larger  number  of  unlike  units,  the  so-called 
ancestral  plasms.  But  this  can  only  continue  until  the 
number  of  the  ancestral  plasms  has  reached  that  of  the 
smallest  units  of  the  entire  hereditary  substance.  These 
units,  originally  quite  alike,  are  so  no  more,  but  each 
possesses  the  tendency  to  transmit,  under  given  condi- 
tions, to  the  new  organism,  the  totality  of  the  character- 
istics of  the  respective  ancestors. 

If  now  sexual  propagation  takes  place  in  a  species 
with  this  kind  of  compound  germ-plasm,  (and  all  living, 
sexually  differentiated  species  must  obviously  have 
reached  this  stage  long  ago),  a  further  multiplication  of 
the  ancestral  plasms  within  the  germ-plasm  can  no  longer 
continue.  Therefore  the  number  of  the  ancestral  plasms 
must  be  reduced  from  time  to  time.  In  the  separation 
of  the  polar  bodies  from  the  egg  before  fertilization,  he 
sees  a  process,  the  result  of  which  is  just  this  reduction.22 

This  reduction  in  the  egg  of  the  number  of  hered- 
itary particles,  as  Weismann  calls  them,  is  obviously  a 
necessary  consequence  of  the  original  assumption  of  the 
uniformity  of  the  germ-plasm.  It  is  very  instructive  that 
two  such  prominent  thinkers  as  Spencer  and  Weismann, 
starting  from  the  same  hypothesis,  have  arrived  at  an 
ancillary  hypothesis  which  is  intrinsically  the  same.  One 
may  well  conclude  from  this  that  whoever  does  not  wish 

21Weismann,  A.  Ueber  die  Zahl  dcr  Richtungskorper,  p.  30. 
1887. 

22Loc.  cit.  p.  32  ff. 


Weismann' s  Ancillary  Hypothesis  55 

to  accept  the  ancillary  hypothesis  must  also  give  up  the 
principle  of  the  uniformity  of  the  germ-plasm. 

Weismann  has  connected  his  theory  in  a  clear  way 
with  the  results  of  cell-study.  He  assumes  that  the  nucleus 
dominates  and  determines  the  nature  of  its  cell,  and  also 
that,  for  all  functions  of  the  cell,  the  material  bearers  of 
the  hereditary  characters  must  be  situated  in. the  nucleus. 
He  assumes  further  that  these  bearers  are  arranged  in 
rows  on  the  chromatin-thread  of  the  nucleus,  and  points 
out  how,  with  this  assumption,  all  the  hereditary  char- 
acters are  divided  through  the  longitudinal  splitting  of 
the  nuclear  skein,  and  how  they  are  distributed  among  the 
two  daughter-cells. 

On  the  basis  of  these  and  similar  conceptions,  he  also 
treats  the  question  concerning  the  cause  of  the  differences 
between  the  single  organs  of  an  individual.  It  is  clear 
that  this  question  forms  a  great  difficulty  of  the  theory. 
For  the  assumption  of  the  ancestral  plasms,  every  one  of 
which  represents  all  the  characters  of  the  individual,  can, 
of  itself,  not  serve  as  an  answer,  especially  in  connection 
with  the  thesis  just  mentioned,  that  the  nature  of  the 
nucleus  determines  the  character  of  its  cell. 

Let  us  see  what  ancillary  hypothesis  Weismann  uses. 
The  theory  of  heredity  demands  that,  on  the  germ- 
tracks,23  the  completeness  of  the  germ-plasm  be  preserved, 
for  every  egg-cell  and  every  bud  contain,  on  the  whole, 
the  same  hereditary  elements  as  the  germ-cells  of  the  pre- 
vious generation.  In  all  the  sequences  of  generations  of 
cells,  which  lead  from  one  egg-cell  to  the  germ-cells  that 
come  next  in  order,  (and  these  are  the  germ-tracks),  the 
germ-plasm  must  therefore  remain  the  same.  In  all  other 
cells,  however,  which  do  not  belong  to  the  organs  capable 

23Cf.  Part  II,  A.  p.  79. 


56       Hypothetical  Bearers  of  Specific  Characters 

of  reproduction,  this,  according  to  Weismann,  need  not  be 
the  case.  On  the  contrary,  from  the  one-sided  differen- 
tiation of  these  cells,  he  believes  that  there  is  a  corre- 
sponding reduction  of  their  germ-plasm.  Every  somatic 
cell  receives,  at  the  time  of  its  origination,  only  those 
hereditary  elements  which  will  be  needed  by  itself  and  its 
descendents. 

Against  this  assumption  objections  have  been  raised 
from  different  sides,  and  some  of  them  we  shall  describe 
in  detail  in  the  Section  on  cellular  pedigrees.  Here,  how- 
ever, we  must  enter  into  the  principal  phase  of  the  ques- 
tion, namely,  the  relation  of  the  ancillary  hypotheses  to 
the  main  principle  of  the  author. 

That  principle  is  the  assumption  of  units,  of  which 
every  one  is  capable  of  reproducing  all,  or  at  least  nearly 
all,  hereditary  characters  of  the  species.  There  is  sup- 
posed to  be,  for  each  individual,  only  one  hereditary  sub- 
stance, only  one  material  bearer  of  the  hereditary  tenden- 
cies.24 To  be  sure,  this  is  composed  of  ancestral  plasms 
which  differ  only  slightly.  A  check  must  necessarily  be 
put  to  an  excessive  accumulation  of  various  hereditary 
tendencies  by  some  kind  of  an  arrangement.  But,  as  we 
have  seen  in  our  first  section,  the  differentiation  of  the 
organs  demands  the  divisibility  of  the  units  of  the  germ- 
plasm,  and  this  in  exactly  the  same  high  degree  that  the 
differences  of  the  individual  organs  and  cells  of  an  or- 
ganism reach  themselves.  In  the  somatic  cells  the  germ- 
plasm  must  therefore  gradually  become  divided  into  those 
components,  and  hence,  these  are  the  bearers  of  the  in- 
dividual hereditary  characters. 

Let  us  continue  to  build  a  few  moments  longer  on  this 
conclusion,  without  reference  to  the  chief  assumption.  In 

2*Ueber  die  Zahl  der  Richtungskorper,  p.  29. 


Nageli's  Idioplasm  57 

that  case  the  germ-plasm  must  evidently  consist,  every- 
where, of  these  same  components,  and,  in  the  lowest 
organisms,  in  which  fertilization  does  not  take  place,  as 
well  as  in  the  germ-cells  of  the  higher  plants  and  animals, 
we  must  assume,  as  the  material  basis  of  heredity,  numer- 
ous material  bearers,  which  correspond  to  the  individual 
hereditary  characters,  and  are  not  inseparably  united.  This 
assumption,  however,  makes  that  of  the  ancestral  plasms 
completely  superfluous.  Thus  it  is  easily  seen  that  the 
whole  ancillary  hypothesis  regarding  an  occasional  nu- 
merical reduction  of  the  ancestral  plasms  may  fail. 

In  a  word:  In  a  consideration  of  the  differentiation 
of  organs,  Weismann's  theory  of  itself  leads  to  the  quite 
opposite  assumption  of  individual  material  bearers  for 
the  individual  hereditary  characters. 

§  7.     Nageli's  Idioplasm 

In  his  mechanico-physiological  theory  of  descent, 
Nageli,  a  few  years  ago,  advanced  the  concept  of  the 
idioplasm25  In  distinction  to  the  other  protoplasms,  it 
is  the  bearer  of  the  hereditary  qualities.  A  factor  (an- 
lage)  representing  every  perceptible  character,  is  present 
in  it;  in  every  individual  of  the  same  species,  even  in 
every  organ  of  a  plant,  it  has  a  slightly  different  compo- 
sition. It  is  not  limited  to  the  nucleus,  but  runs  through 
the  entire  protoplast  as  a  strand  with  many  windings.  All 
cross-sections  of  this  strand  are  alike,  each  one  containing 
every  hereditary  tendency.  That  is  why,  in  cell-division, 
the  daughter-cells,  with  their  part  of  the  strand,  are  also 
endowed  with  all  the  hereditary  factors. 

The  nature  of  the  idioplasm  is  determined  by  its  mole- 

25Nageli,  C.  von.  Mechanisch-physiologische  Theorie  der  Ab- 
stammungslehre.  pp.  21-31.  1884. 


58       Hypothetical  Bearers  of  Specific  Characters 

cular  composition,  and  especially  by  the  arrangement  of 
its  smallest  particles.  These  are  combined  in  hosts,  which 
again  are  united  into  units  of  a  higher  order.  The  latter 
represent  the  primordia  of  the  cells,  tissue-systems,  and 
organs.  The  idioplasm  is  a  rather  solid  substance,  in  l/ 
which  the  smallest  particles  do  not  undergo  any  shifting 
through  the  forces  at  work  in  the  living  organism,  for  it 
is  precisely  the  mutual  arrangement  of  the  molecules  that 
determines  the  nature  of  the  hereditary  factors. 

The  characteristics,  organs,  adaptations,  and  func- 
tions, which  are  all  perceptible  to  us  only  in  a  very  com- 
posite form,  are,  in  the  idioplasm,  resolved  into  their  real 
elements.  These  elements  are  obviously  the  individual 
hereditary  factors,  through  the  manifold  changing  com- 
binations of  which  the  visible  characters  originate.  These 
elements  themselves  are  not  strongly  emphasized  by 
Nageli ;  he  lays  greater  stress  on  the  fact  that  their  prop- 
erties are  conditioned  by  their  molecular  structure,  and 
that  they  themselves,  by  their  mutual  association  with 
each  other,  again  build  up  the  entire  idioplasm. 

No  definite  conclusions  can  be  drawn  from  the  theory 
in  regard  to  the  arrangement  of  the  elements  in  the  idio- 
plasm, nor  in  regard  to  the  question  of  how  the  idioplasm 
develops  its  factors;  here  a  wide  field  is  still  open  to  hy- 
potheses.28 In  general,  however,  the  definite  mutual  ar- 
rangement of  the  elements  forms  the  chief  points  in  which 
Nageli  differs  from  his  predecessors.  Neither  Spencer 
nor  Weismann  enter  into  this  question,  and  Darwin's 
pangenesis  supposes  a  relatively  loose  combination  of 
those  elements,  which  does  not  hinder  a  mutual  penetrat- 
ing and  mixing.  The  question  as  to  how  the  idioplasmic 
strands  of  the  two  parents  unite  during  fertilization  is  also 

26Loc.  dt.  p.  68. 


General  Conclusions  59 

only  briefly  mentioned  by  Nageli,27  and  the  whole  pre- 
sentation of  this  subject  shows  what  great  difficulties  the 
hypotheses  of  the  solid  composition  of  the  idioplasm  en- 
counters. 

Nageli 's  theory  tells  us  as  little  as  any  other  theory 
about  growth  through  assimilation  and  the  multiplication 
of  the  material  bearers  of  heredity.  That  the  properties 
of  those  elements  are  determined  by  their  molecular 
structure  is  just  as  little  an  advantage  of  his  theory;  it  is 
a  conclusion  derived  from  our  most  general  conceptions, 
which  can  be  applied  with  the  same  right  to  the  hypotheti- 
cal units  of  every  theory  of  heredity.  But  how  that  mole- 
cular structure  explains  the  hereditary  factors,  we,  of 
course,  learn  as  little  here  as  by  any  other  thedfy.  It  is  a 
weak  point  of  Nageli's  work  that  these  hitherto  unex- 
plained facts  are  not  clearly  designated  as  such,  and  that 
the  common  basis  of  the  various  theories  is  not  simply 
mentioned  as  such. 

§  8.    General  Considerations 

To  my  mind  the  above  briefly  sketched  theories  clearly 
prove  that  the  fundamental  thought  of  pangenesis,  that 
is,  of  different  material  bearers  for  the  individual  hered- 
itary characters  cannot  be  avoided.  Spencer,  who  wrote 
before  Darwin,  did  not  have  this  thought,  and  it  was  im- 
possible for  him  to  give  a  satisfactory  explanation  of  the 
differentiation  of  organs.  Weismann's  theory,  as  we  have 
already  seen,  led  its  originator  himself  in  that  direction, 
and  forced  him  to  admit,  more  or  less  clearly,. a  divisibility 
of  the  germ-plasm  in  this  sense.  And  Nageli's  idioplasm 
is,  on  the  whole,  built  up  from  those  elements. 

The  more  carefully  we  look  into  these  theories  in  de- 

**Loc.  cit.  pp.  215-220. 


60       Hypothetical  Bearers  of  Specific  Characters 

tail,  the  more  we  shall  find  that  their  efficiency  lies  in  that 
implicitly  made  assumption,  while  their  difficulties  arise 
mostly  through  the  other  hypotheses.  If,  for  the  present, 
we  consider  the  material  bearers  of  the  individual  charac- 
ters, out  of  which  we  must  imagine  the  physiological  units, 
the  ancestral  plasms,  and  the  idioplasm  to  be  composed,  as 
their  elements,  then  the  assumption  of  such  elements  is  in 
itself  sufficient  to  explain  the  fact  of  heredity.  The  pre- 
vailing resemblance  of  children  to  one  of  the  parents,  and 
the  phenomena  of  atavism  become  thereby  comprehensi- 
ble without  any  further  assumptions. 

The  consequence  which  Spencer  and  Weismann  em- 
phasize as  a  necessity  of  their  theory,  namely  the  reduc- 
tion of  the  number  of  units,  (which,  according  to  the 
former,  results  through  mutual  repulsion,  according  to 
the  latter,  through  the  polar  bodies),  is  a  difficulty  which 
arises  from  the  union  of  the  "elements,"  assumed  by  both 
thinkers,  and  not  from  the  assumption  of  the  elements 
themselves.  If  we  discard  the  grouping  of  the  elements 
into  units  or  ancestral  plasms,  such  a  reduction  becomes 
quite  superfluous,  because  the  individual  elements  can  ar- 
range themselves,  after  the  fertilization  in  the  egg,  in  a 
similar  manner  as  previously  in  the  egg  and  in  the  sperm- 
cell.  And  the  phenomena  of  so-called  specific  atavism,  in 
which  species  preserve  latent  characteristics  which  they 
have  inherited  from  their  ancestors,  as,  for  example,  the 
Primula  acaulis  caulescens,  show  that  latent  characters 
need  not  be  thrown  off,  but  may  be  preserved  through 
thousands  of  generations.  In  the  idioplasm  the  firm  union 
of  the  "elements"  is  most  strongly  worked  out,  and  it  is 
precisely  in  that  point  that  every  attempt  fails  to  make  the 
theory  harmonize  with  the  phenomena  of  fertilization  and 
hybridization.  For  these  processes  teach  us  that  hered- 


Similarity  of  Various  Theories  61 

itary  factors  are  miscible,  but  the  idioplasmic  strands  are 
not. 

Variability  teaches  us  that  individual  factors  may  con- 
siderably increase,  independently  from  others,  and, 
on  the  other  hand,  may  almost  completely  disappear.  And 
in  the  formation  of  species  this  possibility  has  been  util- 
ized to  the  highest  degree.  In  the  solid  union  of  the 
idioplasm  such  a  behavior  of  the  individual  "elements" 
might  be  made  extremely  difficult,  if  not  quite  impossi- 
ble. 

We  cannot,  therefore,  maintain  the  solid  union  of  the 
"elements"  into  physiological  units,  ancestral  plasms,  or 
idioplasm.  This  leads,  not  only  in  the  cases  mentioned, 
but  almost  everywhere,  to  contradictions  with  the  facts, 
or  at  least  to  superfluous  assumptions.  But  it  is  just  on 
this  union  that  the  originators  of  these  theories  have  laid 
the  greatest  stress,  while  they  have  nowhere  emphasized, 
as  an  independent  assumption,  the  conception  of  the  "ele- 
ments," and  have  not  considered  that  as  a  thing  apart 
from  their  other  hypotheses. 

As  soon  as  we  do  away  with  this  union,  the  kernel  of 
all  theories  is  the  same  as  that  of  pangenesis,  as  has  al- 
ready been  mentioned  at  the  beginning  of  this  Section. 


CHAPTER  IV 

THE    HYPOTHETICAL    BEARERS    OF   THE    INDIVIDUAL 
HEREDITARY  CHARACTERS 

§  p.     Introduction 

The  views  on  the  nature  of  heredity  expressed  in  the 
first  Section  lead  us  to  the  conviction  that  hereditary 
characters  must  be  units,  independent  to  a  higher  degree, 
and  combined  in  nature  in  the  most  varied  groupings. 

'On  the  other  hand,  a  critical  survey  of  the  theories  so 
far  discussed  induced  us  to  perceive  in  all  of  them  a  more 
or  less  clearly  defined  kernel,  which  assumes  material 
bearers  for  the  individual  hereditary  characters.  To  shell 
this  kernel  was  our  task,  and  it  had  its  justification  in 
those  views.  While  the  solution  of  the  problem  was 
hitherto  achieved  with  difficulty,  this  very  nucleus  is  as 
clear  as  day  in  Darwin's  pangenesis. 

The  assumption  of  different  material  bearers  for  the 
individual  hereditary  characters  was  worked  out  for  the 
first  time  by  Darwin.  The  great  phenomena  of  nature 
which  demand  this  assumption,  and  of  which  I  could 
make  only  a  hasty  sketch  in  the  first  Section,  were  clearly 
comprehended  and  brought  together  in  a  masterful  man- 
ner by  him.  The  entire  work  on  "The  Variation  of  Ani- 
mals and  Plants"  amounts,  so  to  speak,  to  establishing  the 
foundation  of  this  fundamental  idea,  which  he  has  then 
worked  out  and  tried  to  harmonize  with  contradictory 
experiences. 

It  is  remarkable  that  Darwin,  with  a  modesty  that  puts 
us  to  shame,  presents  this  fundamental  thought  as  a  cur- 


Darwin's  Pangenesis  63 

rent  opinion,  and  not  as  his  own  discovery.  He  even 
hoped  to  be  able  to  identify  his  idea  with  Spencer's 
theory.28  But  so  little  did  this  view  prevail  that  his  critics 
have  separated  it  only  in  a  few  instances  from  the  ancil- 
lary hypotheses,  and  most  of  them  have  rejected  the 
fundamental  thought,  together  with  these  secondary  as- 
sumptions. But  let  us  proceed  to  analyze  Darwin's 
theory. 

§  10.     Darwin's  Pangenesis29 

As  already  mentioned  in  the  Introduction,  the  so- 
called  provisional  hypothesis  of  pangenesis  consists,  in  my 
opinion,  of  the  two  following  parts : 

I.  In  the  cells  there  are  numberless  particles  which 
differ  from  each  other,  and  represent  the  individual  cells, 
organs,  functions  and  qualities  of  the  whole  individual. 

These  particles  are  much  larger  than  the  chemical 
molecules,  and  smaller  than  the  smallest  known  organ- 
isms;30 yet  they  are  for  the  most  part  comparable  to  the 
latter,  because,  like  them,  they  can  divide  and  multiply 
through  nutrition  and  growth. 

They  can  remain  latent  through  countless  generations, 
and  then  multiply  only  relatively  slowly,  and  at  some 
later  time  they  may  again  become  active  and  develop  ap- 
parently lost  characte/s  (atavism). 

They  are  transmitted,  during  cell-division,  to  the 
daughter-cells :  this  is  the  ordinary  process  of  heredity. 

II.  In  addition  to  this,  the  cells  of  the  organism,  at 
every  stage  of  development,   throw  off  such  particles, 

28Darwin,  C.  The  Variation  of  Animals  and  Plants.  2:  371,  note. 

29I  have  already  brought  together  the  most  important  parts  of 
this  paragraph  in  the  Introduction  (pp.  3-7)  ;  but  a  repetition  cannot 
be  easily  avoided. 

30Darwin,  C.  loc.  cit.  2:  372. 


64    Hypothetical  Bearers  of  Hereditary  Characters 

which  are  conducted  to  the  germ-cells  and  transmit  to 
them  those  characters  which  the  respective  cells  may  have 
acquired  during  their  development. 

These  two  parts  must  be  considered  separately.  They 
deserve  this  the  more  as  their  significance  has  been  so  far 
generally  misunderstood. 

The  hypothetical  particles  Darwin  called  "gemmules," 
on  account  of  the  analogy  mentioned  in  the  first  proposi- 
tion. This  is  a  poorly  chosen  term,  which  has  contributed 
much  toward  the  raising  of  insurmountable  objections  to 
his  theory.  It  has  led  many  readers  to  imagine  that  they 
were  preformed  germs  (Keimchen)  ;  a  conception  which 
does  not  in  the  least  correspond  to  that  of  Darwin.  On 
the  contrary,  one  would  have  to  say,  according  to  the 
second  proposition,  that  they  originated  only  after  the 
acquisition  of  certain  characters,  or,  at  the  most,  simul- 
taneously with  them.  But  we  will  not  enter  any  further 
into  this  question. 

The  greatest  number  of  investigators,  in  their  criti- 
cisms, have  considered  the  second  proposition  only. 
When  pangenesis  is  mentioned,  only  this  hypothesis  is 
usually  meant.  The  whole  theory  is  identified  with  this 
second  assumption,  and  the  transportation  of  the  gem- 
mules  is  regarded  as  the  chief  point.31 

I  admit  that,  on  a  superficial  'reading,  that  chapter 
might  easily  create  such  an  impression.  But  when  it  is 
read  several  times  attentively,  the  transportation-hypothe- 
sis is  lost  sight  of,  while  the  fundamental  idea,  which  is 
stated  in  the  first  proposition,  becomes  predominant. 

This  is  partly  due  to  the  difficulty  of  familiarizing 
one's  self  immediately  with  the  great  thoughts  of  the 

^Darwin  distinctly  calls  it  "The  chief  assumption."  The  Varia- 
tion of  Animals  and  Plants.  2:  384.  New  York.  1900.  Tr. 


Darwin's  Pangenesis  65 

gifted  investigator,  partly  also  to  the  circumstance,  al- 
ready mentioned,  that  Darwin  himself  represents  the  first 
proposition  as  a  matter  of  course  and  generally  known, 
and  presents  only  the  second  one  as  his  own  hypothesis.32 
The  assumption  of  the  transportation  of  gemmules, 
which  was,  especially  for  plants,  very  greatly  limited  by 
Darwin  himself,  has  been  denied  so  frequently,  and  with 
so  much  ingenuity  that  it  would  be  superfluous  to  criticise 
it  any  further  here.  Especially  to  Weismann  is  the  credit 
due  of  showing  how  little  it  is  demanded  by  well  known 
facts  and  tested  experience.  The  cases  collected  by  Dar- 
win, which  seemed  to  require  it,33  were  exceptions,  and 
their  trustworthiness  has  been  strongly  shaken  by  Weis- 
mann.34 I  believe  I  need  only  cite  here  the  works  of  this 
investigator.35 

'  Freed  from  the  hypothesis  of  the  transmission  of 
gemmules,  pangenesis  now  appears  to  us  in  the  purest 
form.  It  is  the  assumption  of  special  material  bearers 
for  the  various  hereditary  characters.  It  is  true  that 
Darwin  does  not  always  express  himself  clearly  as  to 
what  he  calls  one  hereditary. character,  and  occasionally 

32In  his  letters  also,  he  lays  the  greatest  stress  on  this  part.  Cf. 
Life  and  Letters  of  Charles  Darwin.  3:72-120.  (2:264.  New  York. 
1901.) 

33The  well-known  experiments  of  Brown-Sequard,  which  are  so 
frequently  quoted  as  supporting  the  theory  of  the  heredity  of  ac- 
quired characters,  were  regarded  by  Darwin  himself  as  opposing  his 
hypothesis  of  the  transportation  of  gemmules.  Cf.  Darwin.  The 
Variation  of  Animals  and  Plants.  2:  392. 

34Weismann,  A.  Ueber  die  Vererbung.  1883 ;  also  Die  Bedeutung 
der  sexuellen  Fortpflanzung  fur  die  Selektionstheorie.  p.  93,  etc.  1886. 

85The  so-called  graft-hybrids,  and  the  remarks  on  the  influence 
of  the  male  element  on  the  parts  surrounding  the  germ,  give  no  proof, 
to  my  mind,  of  the  necessity  of  an  assumption  of  transmission.  Cf. 
Part  II,  D,  §  5,  p.  207. 


66    Hypothetical  Bearers  of  Hereditary  Characters 

small  groups  of  characteristics,  or  of  certain  morphologi- 
cal units,  are  probably  regarded  as  such.  This,  however, 
lies  in  the  incompleteness  of  our  knowledge,  which,  in 
certain  cases,  does  not,  even  now,  allow  us  to  carry 
through  the  principle,  even  though  it  is  quite  clear  to  our 
author.  Every  character  which  can  vary  independently^ 
from  others,  must,  according  to  him,  be  dependent  on  a 
special  material  bearer.36 

In  what  manner  these  hypothetical  bearers  are  com- 
bined in  the  cells,  Darwin  has  not  explained.  He  only 
emphasizes  that  each  of  them  can  multiply  independently 
from  the  others,  af  though,  as  the  phenomena  of  variabil- 
ity teach  us,  this  multiplication  frequently  takes  place  sim- 
ultaneously in  small  groups  of  bearers. 

In  the  Introduction  I  have  mentioned  the  reasons 
which  induce  me  to  reject  the  name  "gemmule."  It  is, 
in  everybody's  mind,  too  closely  connected  with  the  trans- 
mission hypothesis.  I  may  be  allowed  to  christen  the 
hypothetical  bearers  of  the  individual  hereditary  predis- 
positions by  a  new  name,  and  call  them  pangens.37 

§  //.     Critical  Considerations 

Among  the  critics  of  Darwin,  Hanstein  deserves  to 
be  named  first,  because  no  other  has  given  as  clear  and 
correct  an  appreciation  of  pangenesis  as  he,  nor  explained 
in  such  a  distinct  manner  the  conclusions  to  which  it 
leads.  Unfortunately,  owing  to  his  particular  turn  of 
mind,  Hanstein38  had  to  discard  these  conclusions,  and 
with  them  the  whole  theory. 


Loc.  cit.  2nd  Ed.    2:  378.    1875. 
37Cf.     Introduction,  p.  7. 

38Hanstein,  J.    Beitrage  zur  allgemeinen  Morphologic  der  Pflan- 
zen.    Bot.  Abhandl  4:     1882. 


Critical  Considerations  67 

Hanstein,  with  good  reason,  first  rejects  the  name 
gemmule,  and  calls  the  Darwinian  units  mikroplasts,  or 
archiplasts.  And  since  he  denies  the  transmission  hy- 
pothesis, he  concludes  from  pangenesis  :39  "One  ought  even 
to  make  the  hypothesis,  that  every  cell  of  the  entire  plam> 
body,  at  its  very  origin,  is  endowed  by  its  mother-cells 
with  every  kind  of  archiplast."40  The  correctness  of  this 
conclusion  will  probably  now  be  admitted  by  all  readers  as 
a  necessary  consequence  of  the  assumption  of  archiplasts, 
as  these  are  indeed  transmitted  from  one  generation  to  the 
other  in  the  egg-  and  sperm-cells.41 

Hanstein's  objections  I  may  here  pass  over.  They 
are  based  chiefly  on  his  conviction  that  it  is  unavoidable 
to  assume  a  special  power  of  nature  for  organisms.42 

Weismann,  in  his  work  on  heredity  (1883.  p.  16), 
has  expressed  himself  against  the  assumption  of  different 
bearers  of  the  individual  hereditary  characters.  Accord- 
ing to  him,  this  conception  does  not  show  how  these 
"molecules"  are  to  stay  together  in  exactly  those  combi- 
nations in  which  they  exist  in  the  germ-plasm  of  the 
respective  species.  Without  doubt  this  is  the  main  diffi- 
culty, and  the  fact  that  it  has  been  the  most  important 
cause  of  the  establishment  of  the  theories  discussed  in 
the  preceding  chapter,  shows  what  weight  it  carries. 

But  this  difficulty  is  no  objection.  It  is  true  that  it 
cannot  be  explained  how  the  individual  pangens  may  be 
held  together.  But  the  more  recent  investigations  on  nu- 
clear division  have  given  us  an  insight  into  extremely 
complicated  processes,  the  object  of  which  is  evidently  an 

™Loc.  cit.  p.  219. 
^Loc.  cit.  p.  223. 
^Loc.  cit.  p.  219. 
**Loc.  cit.  p.  225. 


68    Hypothetical  Bearers  of  Hereditary  Characters 

equitable  distribution  of  hereditary  characters  among  the 
two  daughter-cells.  It  is  not  to  be  thought  that  to-day  we 
already  stand  at  the  end  of  our  investigations  concerning 
the  nucleus.  On  the  contrary,  the  great  discoveries 
which  have  been  made  up  to  the  present  time  awaken 
within  us  the  hope  that  many  more  complex  processes 
within  the  nucleus,  and  of  which  we  have  not,  as  yet,  the 
slightest  inkling,  will  some  time  be  discovered.  The  fact 
that  we  do  not  know  how  the  hypothetical  pangens  are  held 
together  is  in  harmony  with  this  statement.  But  this 
question  does  not  need  to  be  solved  by  auxiliary  hypothe- 
ses. It  is  simply  to  be  reserved  for  further  study  of  the 
phenomena  within  the  protoplasts  and  their  nuclei. 

An  objection  frequently  urged  is  the  necessity  of  as- 
suming such  a  large  number  of  different  pangens.43  Ap- 
parently the  assumption  of  bearers  of  the  whole  specific 
character  is  indeed  much  simpler.  In  that  case  only  one 
hypothetical  unit  is  required  for  each  species.  However, 
if  we  do  not  limit  ourselves  to  the  consideration  of  one 
species,  but  extend  our  view  over  the  whole  world  of  or- 
ganisms, this  objection  breaks  down,  as  has  already  been 
said  in  the  first  Section;  for  we  then  have  to  assume  as 
many  units  as  there  are  and  have  been  species,  and  their 
number  thus  becomes  increased  without  limits.  But  Dar- 
win's units  recur,  most  of  them,  in  numerous  plants  or 
animals,  many  in  almost  all  of  them,  and  a  relatively 
small  number  of  such  hypothetical  pangens  is  sufficient 
to  explain,  through  the  most  varied  possible  groupings, 
all  the  differences  between  species.  On  the  whole,  then, 
the  assumption  of  pangens  is  the  simplest  that  can  be 
made,  and  this  is  obviously  a  great  advantage. 

43Cf.  Weismann,  Die  Bedeutung  der  sexuellen  Fortpflansung,  p. 
102  seq.  1886. 


Conclusion  69 

I  think  I  can  omit  here  a  further  comparison  of  the 
doctrine  of  pangenesis  with  the  theories  established  by 
other  investigators.  Substantially  it  is  contained  in  my 
criticism  of  those  views,  and  besides  it  will  follow  from 
the  working  out  of  the  fundamental  thought  in  the  suc- 
ceeding paragraphs. 

§  12.     Conclusion 

The  considerations  of  the  first  division  of  this  Part, 
and  the  critical  explanations  of  the  second  division,  have 
led  us  to  recognize,  as  unavoidable,  a  hypothesis  of  the 
material  basis  of  hereditary  characters.  It  is,  in  a  cer- 
tain sense,  a  postulate  at  which  everybody  must  more 
or  less  surely  arrive  who  thinks  upon  these  questions, 
and  which  we  have  always  been  able  to  trace  as  the  kernel 
of  the  best  theories  of  inheritance. 

Let  us  conclude  now  by  presenting  this  hypothesis  in 
the  most  simple  manner  possible,  and  by  indicating  the 
most  important  explanations  which  it  is  able  to  give  us 
without  ancillary  hypotheses. 

In  the  first  Division  we  arrived  at  the  conclusion  that 
hereditary  qualities  are  independent  units,  from  the  nu- 
merous and  various  groupings  of  which  specific  charac- 
ters originate.  Each  of  these  units  can  vary  independ- 
ently from  the  others ;  each  one  can  of  itself  become  the 
object  of  experimental  treatment  in  our  culture  experi- 
ments. 

Hereditary  characters  are  connected  with  living  mat- 
ter, and  heredity  depends  on  the  fact  that  children  origi- 
nate from  a  material  part  of  their  parents.  The  visible 
characteristics  of  organisms  are  determined  by  the  invisi- 
ble characters  of  the  living  matter.  In  this  living  substance 
we  assume  special  material  bearers  for  the  individual 
hereditary  characters.  This  is  the  fundamental  thought 


70    Hypothetical  Bearers  of  Hereditary  Characters 

of  Darwin's  pangenfesis,  at  which  almost  all  later  investi- 
gators arrived  more  or  less  clearly.  At  least,  the  critical 
discussion  of  tfieir  opinions  leads,  in  the  end,  to  this 
postulate.  Whether  we  speak  of  the  molecules  of  the  pro- 
toplasm, or  of  the  germ-plasm  and  idioplasm,  as  bearers 
of  the  entire  specific  character;  or  whether  we  place  in 
the  foreground  the  phenomena  of  hereditary;  or,  again, 
whether,  like  Sachs  and  Godlewski,  we  use  as  a  basis  the 
processes  of  growth  and  regeneration,44  we  always  finally 
end  by  assuming  different  bearers  of  the  inherited  attri- 
butes. But  we  reach  this  conclusion  in  the  most  certain 
and  clear  manner  if,  following  Darwin's  example,  we 
regard  the  whole  world  of  organisms  from  the  most 
general  point  of  view  possible. 

According  to  the  hypothesis  concerning  their  nature, 
these  units  have  been  given  different  names.  For  the  one 
adopted  by  me  I  have  chosen  the  name,  pangen. 

These  pangens  do  not  each  represent  a  morphological 
member  of  the  organism,  a  cell  or  a  part  of  a  cell,  but 
each  a  special  hereditary  character.  These  can  be  recog- 
nized by  each  being  able  to  vary  independently  from  the 
others.  Their  study  opens  a  very  promising  field  to  ex- 
perimental investigation. 

The  pangens  are  not  chemical  molecules,  but  morpho- 
logical structures,  each  built  up  of  numerous  molecules. 
They  are  the  life-units,  the  characters  of  which  can  be 
explained  in  an  historical  way  only. 

We  must  simply  look  for  the  chief  life-attributes  in 
them,  without  being  able  to  explain  them.  We  must 
therefore  assume  that  they  assimilate  and  take  nourish- 

44Sachs,  J.  Stoff  und  Form  der  Pflanzenorgane.  Arbeit.  Bot. 
Instit.  Wiirzburg.  2:  452.  1880.  Godlewski,  E.  Bot.  Centralb.  34: 
82.  1888. 


Conclusion  71 

met  and  thereby  grow,  and  then  multiply  by  division, 
two  new  pangens,  like  the  original  one,  usually  originat- 
ing at  each  cleavage.  Deviations  from  this  rule  form  a 
starting  point  for  the  origin  of  varieties  and  species. 

At  each  cell-division  every  kind  of  pangen  present  is, 
as  a  rule,  transmitted  to  the  two  daughter-cells.  What 
combination  of  circumstances  is  the  condition  of  this,  and 
what  relation  .is  established  by  the  practically  uniform 
multiplication  of  the  various  pangens  of  an  individual, 
we  do  not  know. 

The  pangens,  in  smaller  and  larger  groups  must  stand 
in  such  a  relation  to  each  other  that  the  members  of  one 
group,  as  a  rule,  become  active  at  the  same  time.45 

All  these  conclusions  follow  naturally  when  we  try 
to  connect  the  fundamental  thought  with  the  known 
phenomena  of  heredity  and  variability. 

The  whole  import  of  this  fundamental  idea  will,  I 
believe,  be  made  most  clear  by  briefly  grouping  now  the 
most  important  advantages  of  the  hypothesis  in  answering 
some  great  biological  questions.  For  entire  large  groups 
of  phenomena  are  made  comprehensible  to  us  in  a  simple 
manner,  and  this  without  any  ancillary  hypothesis,  by  a 
mere  consideration  of  the  ever  changing  relative  quan- 
tities in  which  the  pangens  must  occur,  according  to  the 
nature  and  age  of  the  cells.  In  the  main  these  advan- 
tages have  already  been  pointed  out  by  Darwin. 

According  to  Darwin's  idea,  the  phenomena  of  hered- 
ity evidently  depend  on  the  fact  that  the  living  matter 
of  the  child  is  built  up  of  the  same  pangens  as  those 
of  its  parents.  If  the  pangens  of  the  father  predominate 
in  the  germ,  the  child  will  resemble  him  more  than  the 

45Darwin  called  these  groups  "compound  gemmules.'  Loc.  tit. 
2:  366.  New  York.  1900. 


72    Hypothetical  Bearers  of  Hereditary  Characters 

mother,  if  only  certain  pangens  of  the  father  prevail, 
then  this  resemblance  will  be  limited  to  single  character- 
istics. If  certain  pangens  are  fewer  in  number  than 
others,  then  the  character  represented  by  them  is  only 
slightly  developed;  if  they  are  very  few,  the  character 
becomes  latent.  If  external  conditions  cause  later  a  rela- 
tively great  increase  of  such  pangens,  the  previously 
latent  character  reappears,  and  we  observe  a  case  of 
atavism.  If  certain  pangens  entirely  cease  multiplying, 
the  respective  character  is  definitely  lost,  but  this  seems 
to  occur  very  rarely. 

In  the  protoplasm,  or  at  least  in  the  nuclei,  of  the 
egg-  and  sperm-cells,  as  well  as  in  that  of  all  buds,  all 
the  pangens  of  the  respective  species  are  represented; 
every  kind  of  pangen  in  a  definite  number.  Predominat- 
ing characters  correspond  to  numerous  pangens,  slightly 
developed  attributes  to  less  numerous  ones. 

The  differentiation  of  the  organs  must  be  due  to  the 
fact  that  individual  pangens  or  groups  of  them  develop 
more  vigorously  than  others.  The  more  a  certain  group 
predominates,  the  more  pronounced  becomes  the  char- 
acter of  the  respective  cell.  Connected  with  this  is  the 
fact  that  external  influences  may  frequently  alter  the 
character  of  an  organ  in  its  earliest  youth,  but  that  this 
becomes  more  difficult  the  more  advanced  it  is  in  its 
development,  i.  e.,  the  more  strongly  definite  pangens 
are  already  predominating. 

The  regeneration  of  detached  members,  the  restora- 
tion of  smaller  lost  parts  of  tissues,  and  the  closing  up 
of  wounds  are  evidently  due  to  the  fact  that  the  pangens 
of  the  lost  parts  are  not  limited  to  these  parts,  but  that 
all  cells  capable  of  reproduction  contain  all  the  pangens 
necessary  thereto. 


Basis  of  Systematic  Relationship  73 

Some  pangens  represent  characters  which  usually  de- 
velop only  in  quite  definite  organs.  If  these  happen  to- 
predominate  in  the  wrong  place  we  get  the  phenomena  of 
metamorphosis.46  If,  for  example,  the  pangens  which 
determine  the  peculiarities  of  the  petals  develop  in  the 
bracts  the  petalody  of  the  bracts  takes  place. 

Other  pangens  represent  qualities  which  may  appear 
in  many  or  in  all  members  of  the  plant.  And  therein  lies 
doubtless  the  reason  that  such  characters  are  so  very 
often  equally  strongly  or  feebly  developed  in  all  of 
those  members.  Thus  the  red  coloring  matter  of  the 
white-flowered  varieties  of  red  species  is  most  frequently 
also  lacking  in  the  stem  and  foliage,  and  plants  with 
variegated  leaves  not  infrequently  bear  variegated  fruit. 

Phenomena  of  correlative  variability,  when  not  of 
purely  historical  nature,  i.  e.,  if  not  originated  by  simul- 
taneous accumulation  of  two  independent  qualities,  find 
their  explanation  in  the  union  of  the  pangens  into  groups. 

Systematic  relationship  is  based  on  the  possession  of 
like  pangens.  The  number  of  identical  pangens  in  two 
species  is  the  true  measure  of  their  relationship.  The 
work  of  the  systematist  should  be  to  make  the  applica- 
tion of  this  measure  possible  experimentally,  by  finding 
the  limits  of  the  individual  hereditary  characters.  Sys- 
tematic difference  is  due  to  the  possession  of  unlike  pan- 
gens. 

According  to  pangenesis,  there  may  be  two  kinds  of 
variability.  These  are  differentiated  in  the  following 
manner  by  Darwin.47  In  the  first  place  the  pangens 
present  may  vary  in  their  relative  number,  some  may  in- 
crease, others  may  decrease  or  disappear  almost  entirely, 

46Darwin,  C    Loc.  cit.  2:    387. 
"Loc.  cit.  p.  390. 


74    Hypothetical  Bearers  of  Hereditary  Characters 

some  that  have  long  been  inactive  may  resume  activity, 
and  finally  the  grouping  of  the  individual  pangens  may 
possibly  change.  All  of  these  processes  will  amply  ex- 
plain a  strongly  fluctuating  variability. 
^>  In  the  second  place  some  pangens  may  change  their 
nature  more  or  less  in  their  successive  divisions  or,  in 
other  words,  new  kinds  of  pangens  may  develop  from 
those  already  existing.  And  when  the  new  pangens,  per- 
haps in  the  course  of  several  generations,  gradually  in- 
crease to  such  an  extent  that  they  can  become  active,  new 
characters  must  manifest  themselves  in  the  organism. 
\  In  a  word :  An  altered  numerical  relation  of  the  pan- 
gens already  present,  and  the  formation  of  new  kinds  of 
pangens  must  form  the  two  main  factors  of  variability.48 
Unfortunately  we  have  not  yet  succeeded  in  analyzing 
the  observed  variations  so  far  as  to  be  able  to  determine 
the  share  of  each  of  those  factors.  But  it  is  clear  that 
the  former  kind  is  more  likely  to  determine  the  individual 
differences  and  the  numberless  small,  almost  daily  varia- 
tions and  monstrosities,  while  the  second  one  has  chiefly 
to  produce  those  variations  on  which  depends  the  grad- 
ually increasing  differentiation  of  the  entire  animal  and 
vegetable  world. 

This  conception  of  phylogenetic  variability  indicates 
that  the  pangens,  too,  must  have  their  pedigrees  which 
correspond  to  the  pedigrees  of  the  respective  character- 
istics. At  every  advance  in  the  pedigree  of  the  species 
one  or  more  new  kinds  of  pangens  must  have  developed 
from  those  present.  In  the  lowest  organisms,  therefore, 
the  pangens  themselves  become  relatively  simple,  and  not 

48In  a  note  to  the  translator,  the  author  says:  "That  sentence 
has  since  become  the  basis  of  the  experiments  described  in  my  'Mu- 
tationstheorie.' "  Tr. 


Conclusion  75 

very  different  from  each  other.  With  increasing  dif- 
ferentiation they  must  themselves  have  become  more 
complicated,  and  gradually  more  unlike  each  other. 

But  the  farther  we  get  away  from  the  facts  the  more 
likely  we  are  to  get  lost  in  false  speculations.  My  object 
was  only  to  place  the  fundamental  idea  of  Darwin's  pan- 
genesis  in  the  right  light.  I  hope  I  have  succeeded  in 
this. 


OF    THE 

:UTY 

OF  / 


PART  II 

INTRACELLULAR  PANGENESIS 
A.    CELLULAR  PEDIGREES 


CHAPTER  I 

THE    RESOLVING    OF   INDIVIDUALS    INTO    THE    PEDI- 
GREES OF  THEIR  CELLS 

§  i.     Purpose  and  Method 

Since  the  founding  of  the  cell-theory  by  Schleiden  and 
Schwann,  cells  have  come  more  and  more  to  the  fore- 
ground of  anatomical  and  physiological  consideration. 
The  theory  of  heredity,  also,  which  about  two  decades 
ago  was  hardly  at  all  in  touch  with  the  cell-theory,  has 
given  up  this  isolated  position,  and  sees  in  the  more  re- 
cent investigations  on  cell-division  and  the  process  of 
fertilization  an  important  furtherance  of  its  problems. 

Oninis  cellula  e  cellula.  Not  only  does  this  saying 
dominate  microscopic  science,  but  it  is  steadily  rising  into 
supreme  command  over  all  Biology.  That  every  cell  has 
originated  from  a  material  part  of  its  mother-cell,  and 
that  it  owes  its  specific  characters  to  this  origin,  is  now 
accepted  in  the  theory  of  heredity  as  the  basis  of  all 
thorough  considerations.  Whether  or  not  this  source  is 
sufficient  for  the  explanation  of  all  phenomena  was  the 
question  which  induced  Darwin  to  formulate  his  pan- 
genesis.  And  this  question  remains  the  first  to  be  an- 
swered with  reference  to  every  new  group  of  facts  ap- 
pearing within  the  domain  of  heredity. 

The  phenomena  known  at  present,  at  least  in  so  far 
as  they  have  been  sufficiently  thoroughly  investigated, 
demand  an  affirmative  answer  to  that  question.  This 
was  conclusively  demonstrated  by  Weismann,  as  has  been 


80  Cell-Pedigrees 

already  mentioned  in  the  first  Part.  We  need  therefore 
not  deal  with  that  question  in  this  Section. 

Not  the  organisms,  but  the  cells,  are  therefore  the 
units  of  the  theory  of  heredity.  One  has  to  go  back  to 
these  for  a  clear  understanding.  In  the  practical  pedi- 
grees of  the  animal-  and  plant-breeders  of  course  only 
the  individuals  figure,  but  for  a  scientific  insight,  these  are 
not  sufficient,  as  is  well  known  to  the  greatest  authorities 
among  breeders. 

Here  the  germ-cells  (egg-  and  sperm-cells)  come  into 
the  foreground  for  consideration.  They  are  the  material 
parts  of  the  parents  from  which  the  children  issue,  and 
hence  form  the  material  bond  between  the  successive 
generations.  For  every  genii-cell  we  may  trace  a  series 
of  ancestral  cells  back  to  the  last  preceding  generations. 
In  this  way  we  may  proceed  further,  and  follow  up  the 
pedigree  of  the  germ-cells  through  a  series  of  generations. 
The  great  scientific  significance  of  these  sequences  of  cells 
has  been  strongly  emphasized  by  Weismann;  they  form, 
without  doubt,  the  basis  for  the  theory  of  cell-pedigrees. 

But  this  kind  of  treatment  Jeads  to  a  one-sided  con- 
ception of  the  problem.  We  ought  rather  to  trace  the 
ancestral  line  of  all  the  cells  of  the  entire  body  back 
to  the  first  cell  from  which  the  organism  started.  It  is 
true  that  thereby  the  task  becomes  much  more  extensive 
and  complicated,  and  it  is  a  question  whether  a  sufficient 
anatomical  and  ontogenetic  basis  is  at  hand  for  its  solu- 
tion. Nevertheless  it  is  only  in  this  way  that  we  can 
approach  a  uniform  treatment  of  the  subject,  and  group 
the  available  facts  in  such  a  way  that  they  do  not  de- 
ceive us,  nor  lead  us  to  an  overestimation  of  the  signifi- 
cance of  isolated  cell-sequences  selected  by  us  arbitrarily. 

We  should,  therefore,  trace  out  the  pedigree  of  the/ 


Cell-Pedigrees  81 

individual  cells  for  the  whole  organism.  Or,  in  other 
words,  we  should  resolve  the  individual  into  its  cells  and 
and  their  lineage.  To  this  end  the  history  of  develop- 
ment must  furnish  us  the  requisite  facts  which,  however, 
must  include  all  forms  of  reproduction. 

The  cellular  pedigrees  that  are  to  be  traced  are  of  a 
purely  empirical  nature.  As  Sachs  has  already  empha- 
sized, we  have  but  to  record  the  facts  in  as  simple  a  group- 
ing as  possible,1  and  see  what  conclusions  can  be  drawn 
from  them  without  resorting  to  any  hypothesis.  The 
harvest  will,  to  my  mind,  be  much  richer  than  would  be 
imagined  at  first  glance. 

That  the  chief  results  of  the  consideration  of  cellular 
pedigrees  in  both  the  plant  and  animal  kingdoms  will  lead 
to  the  same  general  conclusions,  probably  no  one  doubts  at 
present.  But  the  conditions  are  quite  different  in  the 
plant  world  from  those  in  the  animal  kingdom.  The  vari- 
ous kinds  of  reproduction  in  the  latter  are  not  nearly 
as  numerous  as  in  the  former.  A  study  of  animals  is 
therefore  much  more  exposed  to  the  danger  of  one-sided 
treatment  than  that  of  plants.  Moreover,  with  the  bot- 
anist, the  conviction  that  the  anatomical  and  ontogenetic 
investigation  should  always  penetrate  at  least  to  the 
individual  cells  has,  under  the  influence  of  Mohl  and 
Nageli,  for  almost  half  a  century,  taken  much  deeper  root. 
Accordingly  the  ancestral  sequence  of  by  far  the  greatest 
number  of  cells  is,  in  innumerable  cases,  if  not  without 
gaps,  demonstrable  with  sufficient  certainty  at  least  in  its 
main  lines. 

Therefore  I  shall  be  able  to  limit  myself  in  this  sec- 
tion, without  danger,  to  the  cellular  pedigrees  of  plants. 
And  this  the  more  so,  as  the  most  important  lines  of 

1  Sachs,  J.  von.  Vorlesungen  uber  Pflansenphysiologie.    1882. 


82  Cell-Pedigrees 

those  pedigrees  have  lately  been  frequently  emphasized 
Afor  the  animal  kingdom  by  Weismann  and  others,  and  a 
comparison  of  both  kingdoms  with  reference  to  this 
point  does  not,  therefore,  offer  any  considerable  diffi- 
culties. 

§  2.     The  Cellular  Pedigrees  of  the  Homoplastids 

In  unicellular  species  the  pedigrees  of  the  individuals 
coincide  with  the  cellular  pedigrees.  But  such  is  also 
the  case  with  those  organisms  of  few  cells,  the  cells  of 
which  are  as  yet  quite  alike  and  not  organized  for  various 
functions.  The  Oscillariae  are  many-celled  threads,  but 
all  the  cells  are  alike,  every  one  of  them  is  equally  able 
to  propagate  the  species.  Gotte  has  named  such  organ- 
isms homoplastids,  as  compared  with  the.heteroplastids, 
the  cells  of  which  are  adapted  for  various  functions. 

It  is  clear  that  the  ancestral  trees  of  cellular  descent  of 
the  homoplastids  are  entirely  composed  of  like  branches. 
It  depends  only  upon  external  circumstances,  and  the 
struggle  for  existence,  which  of  the  cells  will  become  new 
individuals,  and  which  branches  of  the  family  tree,  there- 
fore, will  continue  the  descent  through  the  series  of  gen- 
erations. 

In  the  higher  plants  and  animals,  on  the  contrary, 
only  definite  branches  of  the  cellular  pedigree  lead,  in  the 
normal  course  of  development,  to  the  cells  that  begin 
the  next  generation,  the  other  branches  being  already  ex- 
cluded, by  their  nature,  from  taking  part  in  the  normal 
propagation  of  the  species.  The  branches  of  the  tree  are 
here,  therefore,  not  only  morphologically  different,  but 
also  intrinsically  unlike  in  their  relation  to  the  pedigree 
of  the  species. 

The  differentiation  of  the  cellular  pedigrees  started 


Cell-Pedigrees  83 

with   the   development   of  the   heteroplastids    from  the/ 
homoplastids.    The  undifferentiated  cellular  pedigrees  of 
the  latter  do  not  afford  us  any  clue  for  judging  the  phe- 
nomena of  heredity.    Hence  we  leave  them  aside,  and 
turn  our  attention  entirely  to  the  heteroplastids. 

§  j.     The  Cellular  Pedigree  of  Eqidsctum 

Before  we  begin  describing,  at  least  in  their  main 
lines,  the  extremely  complex  cellular  pedigrees  of  the 
higher  plants,  we  will  elucidate  the  whole  method  with  a 
rather  simple  example.  I  choose  for  the  purpose  the  genus 
of  the  horsetails  (Equisetum).  Their  cellular  pedigree 
belongs,  in  spite  of  their  alternation  of  generations,  to  the 
simplest  that  are  to  be  found  among  the  leaf-forming 
plants,  or  Cormophytes.  There  are  two  ways  of  arriving 
at  a  conception  of  the  main  lines  of  the  picture.  One  of 
them  is  the  progressive,  the  other  the  retrogressive.  The 
first  one  follows  up  the  track  of  ontogeny,  the  second 
one  descends  in  the  opposite  direction.  If  one  is  inter- 
ested in  deciphering  the  combination  for  all  the  cells  of 
one  plant,  then  the  first  method  is  obviously  the  simplest 
and  the  safest.  But,  in  choosing  it,  the  relative  value  of 
the  two  new  twigs,  into  which  the  stem  divides,  can  only 
be  judged  when  the  ends  of  both  twigs  are  constantly  and 
simultaneously  kept  in  view.  But,  in  tracing  only  the 
main  lines  of  the  picture,  it  is,  in  most  cases,  much  more 
convenient  to  choose  the  opposite  direction.  For,  in  the 
retrogressive  direction,  all  paths  evidently  lead  back  to 
the  egg-cell,  so  that  in  this  direction  no  erring  is  ever  to 
be  feared. 

I  assume  that  through  a  combination  of  both  methods 
the  picture  of  the  cellular  pedigree  of  an  Equisetum- 
species,  e.  g.  of  E.  palustre  has  been  developed  and  lies 


84  Cell-Pedigrees 

before  us.2  The  fertilized  egg-cell  in  the  archegonium 
begins  its  growth  by  divisions,  the  first  of  which  stands 
nearly  at  right  angles  to  the  axis  of  the  archegonium ; 
this  is  followed  by  two  walls  at  right  angles  to  this  and  to 
themselves.  From  the  lower  octants  develop  the  root  and 
the  foot  of  the  young  sporophyte,  the  latter  by  the  for- 
mation of  a  small-celled  tissue  body  due  to  continued  di- 
visions. These  branches  of  the  pedigrees  are  thus  ended. 
From  one  of  the  upper  octants  of  the  embryo  the  apical 
cell  of  the  first  shoot  originates,  the  other  octants  partici- 
pate in  the  formation  of  the  annular  thickening  which 
represents  the  first  leaf-whorl,  and  thus  soon  end  their 
growth,  after  continued  divisions. 

The  growth  of  the  first,  as  well  as  of  all  successive 
shoots  is  dominated  by  the  apical  cell.  The  latter  occu- 
pies the  apex  of  the  shoot,  its  upper  cell-wall  is  spheri- 
cally arched,  while  downward  it  is  limited  by  three  almost 
plane  walls.  It  has,  therefore,  the  shape  of  an  inverted 
three-sided  pyramid.  It  divides  only  by  walls  which  run 
parallel  to  the  three  sides  of  the  pyramid ;  every  detached 
piece  is  called  a  segment.  By  numerous  divisions,  the 
three  successive  segments,  parallel  to  the  three  sides 
of  the  pyramid,  always  form  an  internode  with  a  leaf- 
whorl  at  its  upper  end.  The  whole  shoot,  therefore, 
consists  of  sections  each  of  which  owes  its  origin  to  a 
segment  whorl  of  the  apical  cell. 

The  apical  cell,  therefore,  evidently  represents  the 
main  stem  of  our  pedigree;  every  segment  corresponds 
to  a  branch.  During  the  development  of  the  shoot,  and 
consequently,  during  the  first  year  ®f  vegetation  of  the 

Illustrations  of  the  required  stages  of  development  are  found  in 
Goebel,  K.  Grundzuge  der  Systematik  und  Speziellen  Pflanzenmor- 
phohgie  pp.  286-304.  1882. 


Significance  of  the  Apical  Cell  85 

individual,  the  main  stem  remains  simple,  and,  since  the 
first  shoot  never  bears  a  sporophore  without  modification 
of  its  activity,  it  ends  witl}  the  death  of  the  shoot  at  the 
end  of  the  'first  summer. 

Each  segment  that  separates  from  the  apical  cell  di- 
vides first  into  an  upper  and  a  lower  half;  these,  through 
further  walls,  into  a  body  of  tissue,  from  which  now  all 
the  cells  of  the  respective  part  of  the  internode  and  the 
leaf-whorl  arise.  The  sequence  of  division  has  been  ex- 
plained by  Cramer  and  Rees  and  can  be  found  in  the 
Lehrbuch  der  Botanik,  of  Sachs  and  Goebel.  Further- 
more, there  should  be  emphasized,  first  of  all,  the  fact 
that,  in  the  outer  cell-layer  of  the  body  of  tissue,  and 
alternating  with  the  teeth  of  the  leaf-blade,  favored  cells 
are  formed,  each  of  which  can  grow  into  a  lateral  shoot. 
The  green  shoots  of  older  plants  as  a  rule  actually  bear, 
in  every  leaf-whorl,  a  circle  of  as  many  branches  as  the 
whorl  has  members.  But,  in  the  first  shoot,  they  usually 
do  not  reach  development.  Every  lateral  bud,  when  de- 
veloping into  a  shoot,  possesses  an  apical  cell,  which  starts 
the  development  of  the  branch  in  the  same  manner  as  the 
terminal  cell  of  the  main  shoot. 

Thus  in  every  branch  the  apical  cell  again  forms  the 
main  line  of  the  pedigree.  It  is  true  that  this  line  does 
not  join  the  main  stem  in  a  simple  manner  but  it  can  be 
clearly  traced  back,  through  the  first  divisions  of  the 
segment,  to  the  stem.  Now  every  segment,  and  within 
it,  during  their  first  cleavages,  those  cells  from  the  later 
divisions  of  which  the  apical  cells  of  the  lateral  branches 
arise,  we  shall  regard  as  the  main  stem  of  our  pedigree. 
All  other  cell-sequences  will  be  considered  as  lateral 
branches,  for  only  in  this  manner  can  we  get  a  clear 
picture. 


86  Cell-Pedigrees 

Let  us  return  now  to  the  shoot  during  its  first  year  of 
vegetation.  At  the  end  of  the  summer  it  perishes.  A 
lateral  bud  in  one  of  the  basal  leaf-whorls,  however,  con- 
tinues to  live,  and  develops  during  the  next  year  into  a 
new  shoot,  which  grows  stronger  and  larger  than  the  first 
one,  but  does  not  yet  bear  any  organs  of  fructification. 
This  course  continues  for  several  years,  until  the  plant 
has  become  quite  vigorous.  Sometimes  the  third  or  one 
of  the  following  shoots  grows  downward  into  the  ground, 
to  form  the  rhizome,  which,  from  now  on,  forms  the 
main-shoot  of  the  plant,  branching  beneath  the  ground 
and  sending  up  into  the  air  the  leaf-bearing  and  spore- 
bearing  shoots.  These  are  distinct  in  Equisetum  arvense 
and  some  other  species.  In  the  spring  the  pale,  fertile 
unbranching  shoots  arise,  in  the  summer  the  extensively 
spreading,  green  but  sterile  branches. 

The  cellular  pedigree  of  the  whole  large  plant  would 
very  soon  present  an  inextricable  picture.  To  avoid  this 
danger,  we  must  mark  especially  the  main  lines,  perhaps 
by  indicating  them  by  heavier  marks.  We  must  also 
draw  the  lines  as  straight  as  possible.  Supposing  all  of 
this  executed,  we  get  a  pedigree  of  the  apical  cells  which 
in  the  picture  stands  out  clearly  as  a  connected  system, 
and  to  which  all  the  rest  is  laterally  added.  We  shall 
call  the  lines  of  the  pedigree  of  the  apical  cells  the 
branches,  the  other  ramifications  the  twigs.  In  order  to 
avoid  misunderstandings,  it  must  be  remembered,  that 
the  pedigree  of  apical  cells  does  not  consist  exclusively 
of  apical  cells,  since  these  do  not  originate  directly  from 
each  other. 

According  to  this  definition  the  development  of  the 
twigs  of  the  pedigree  is  always  limited,  only  in  the 
branches  resides  the  ability  of  new  ramifications,  and 


Cell-Pedigrees  87 

thence  of  a  continuation  of  the  main-lines.  But  this  is 
not  the  case  to  the  same  extent  for  all  branches  as  we  shall 
soon  see. 

In  our  picture  two  important  parts  are  still  lacking, 
one  of  them  being  the  roots,  the  other  the  organs  of  re- 
production. The  roots  need  only  briefly  be  mentioned. 
They  grow  by  means  of  apical  cells,  the  same  as  the 
shoots,  and  are  present  in  the  lateral  buds  before  the 
latter  arise  from  the  leaf  whorls.  As  a  rule,  every  bud 
at  first  forms  only  one  root,  which  develops  from  an  inner 
cell,  situated  on  its  under  side.  This  cell  becomes  the 
apical  cell  of  the  young  root.  Therefore,  in  the  genea- 
logical tree  every  root,  as  well  as  every  shoot,  is  repre- 
sented by  a  branch  with  its  numerous  twigs.  But  since 
the  roots  never  bear  leaf -buds,  as  in  many  ferns  and  pha- 
nerogams, and  therefore  never  produce  any  organs  of 
reproduction,  they  are  always  only  sterile  branches  of  the 
pedigree. 

In  the  case  of  Equisetum  arvense  this  is  the  fate  of 
by  far  the  greater  portion  of  the  branches  of  the  cellular 
pedigree.  Because  here  only  the  pale,  yellow  shoots  of 
the  later  years,  without  chlorophyll,  are  selected  for  re- 
production. Thus,  here  too,  we  distinguish  sterile  and 
fertile  branches. 

At  the  apex  of  the  fertile  shoots  stand  the  sporangia 
in  crowded  spikes  of  four-  to  six-sided  shields,  which  have 
their  stems  in  the  center.  Every  one  of  these  corres- 
ponds in  its  origination  to  a  tooth  of  a  leaf-whorl.  Hence, 
the  cell-pedigrees  of  the  individual  shields  can  be  derived 
in  a  similar  manner  from  the  apical  cell  of  the  shoot,  as 
in  the  vegetative  part ;  and  in  the  same  way  the  origin  of 
each  single  spore  can  be  traced  back  to  it.  These  lines 
again  we  call  branches,  while  all  the  lines  leading  to  the 


88  Cell-Pedigrees 

other  cells  of  the  sporangial  tissues  must  be  regarded  as 
twigs.  For  here,  too,  the  branches  possess  the  power  of 
continuing  the  pedigree,  but  the  twigs  do  not. 

On  germination  the  spores  produce  the  male  and  the 
female  prothallia.  The  former  bear  only  the  male  sexual 
organs  or  antheridia,  the  latter  only  the  female  organs 
or  archegonia.  In  the  cell-pedigrees  we  again  imagine 
heavy  straight  lines  for  those  cell-sequences  which  lead  to 
the  egg-cells  and  to  the  spermatozoids.  These  represent 
for  us  the  branches,  all  the  others  the  twigs. 

We  have  arrived  at  the  completion  of  our  sketch,3 
since  we  have  been  through  the  much  ramified  path  from 
the  fertilized  egg-cell  to  the  new  germ-cells,  and  have 
taken  in  its  numerous  side-paths.  Let  us  glance  once 
more  over  the  whole,  and  we  shalK  see  that,  by  empha- 
sizing the  branches  instead  of  the  twigs  we  have,  in  spite 
of  the  great  complication  a  simple  and  clear  picture.  For 
the  branches  again,  we  have  to  make  a  distinction  be- 
tween the  fertile  and  the  sterile.  Only  the  former  lead 
finally  to  egg-cells,  or  to  spermatozoids,  i.  e.,  to  new  in- 
dividuals; the  sterile  branches  do  not  do  this.  Funda- 
mentally, then,  they  behave  towards  the  fertile  ones  like 
the  twigs ;  they  take  no  part  in  the  pedigree  of  the  species. 


§  4.     The  Main  Lines  in  the  Cell-Pedigrees 

For  those  cell-sequences,  which  in  the  cell-pedigree 
lead  from  the  fertilized  egg-cell  through  the  individual 
to  the  next  generation,  I  may,  as  a  continuation  of  Weis- 

3In  order  not  to  complicate  the  illustration  I  have  not  discussed 
here  the  vegetative  multiplication.  I  shall  come  back  to  it  in  the  next 
Section. 


Germ-Tracks  and  Somatic  Tracks  89 

matin's  clear  statements  employ  the  name  germ-track. 
This  conception  would  then  correspond  exactly  to  the 
fertile  branches  of  the  cell-pedigree  in  the  illustration 
selected  above.  We  shall,  in  the  future,  keep  this  shorter 
designation  for  it,  and  in  contradistinction  we  shall  call 
all  other  sequences  of  generations  of  cells,  the  sterile 
branches  as  well  as  the  twigs  of  our  illustration,  the 
somatic  tracks. 

A  germ-track  then,  always  leads  in  our  cell-pedigree 
from  the  fertilized  egg-cell  to  the  new  egg-  or  sperm-cell ; 
we  imagine  it  drawn  very  straight  and  clear  in  our  dia- 
gram. Somatic  tracks  begin  at  all  points  of  the  germ- 
tracks  and  lead,  constantly  branching,  to  all  the  vegeta- 
tive cells  of  the  body.  The  cells  which  are  situated  on 
the  germ-tracks,  can  be  called  germ-track-cells  or,  accord- 
ing to  Jager,  phylogenetic,  or  perhaps  still  more  distinct- 
ively, phyletic  cells.  They  are  thus  sufficiently  distin- 
guished from  the  ontogenetic  or  somatic  cells. 

It  is  a  matter  of  course  that  the  distinctions  intro- 
duced here,  and  therefore  also  the  names  and  their  defi- 
nitions, are  of  a  purely  descriptive  nature.  There  can  be 
no  question  as  to  their  correctness  since  they  are  quite 
arbitrary.  The  question  is  only,  are  they  practical,  i.  e., 
can  they  lead  us  to  a  clear  insight. 

We  must  not  wish  to  substitute  a  theoretical  meaning 
for  the  conception  of  the  germ-tracks.  Otherwise  the 
definition  would  not  be  sufficiently  clear.  Therefore 
Weismann's  germ-cells  correspond  only  in  their  main 
features,  and  not  everywhere,  with  our  germ-track  cells. 
This  is  especially  shown  by  the  circumstance  that,  ac- 
cording to  his  theory,  sexual  cells  are  frequently  produced 
by  somatic  cells,  and  that  he  devotes  a  detailed  discussion 
to  the  fact  that  the  splitting  off  occurs  a  little  sooner  in 


90  Cell-Pedigrees 

some  groups  of  the  animal  kingdom  and  a  little  later 
in  others.4 

In  my  picture,  however,  sexual  cells  are  never  pro- 
duced by  somatic  ones,  but  the  main  lines  are  always 
drawn  through  the  ancestral  rows  of  the  germ-cells.  Ac- 
cordingly these  produce  all  the  somatic  rows  of  cells. 
We  see  that  it  is  merely  a  matter  of  choosing  the  main 
lines  for  the  picture,  and  not  of  a  comprehension  of  the 
facts.  But  with  my  choice  the  picture  becomes  simple 
and  clear,  and  essentially  the  same  for  plants  as  for  ani- 
mals. To  my  mind  the  germ-cells  of  the  hydroids  and  of 
the  phanerogams  are  not,  as  Weismann  assumes,5  secreted 
by  the  Metazoon  itself,  but  are  formed,  as  in  the  case  of 
all  other  sexually  differentiated  heteroplastids,  on  the 
germ-tracks,  only  the  number  of  cell-divisions  which  pre- 
cede their  origin  on  this  track  is  here  very  great. 

According  to  my  definition,  a  germ-track  never  origi- 
nates from  a  somatic  track.  A  continuity  of  the  germ- 
cells  does  not  occur  as  a  very  rare  case,6  but  everywhere, 
and  without  exception,  although  sometimes  at  a  great 
distance,  along  the  germ-track.  The  whole  question  of 
whether  somatic  plasm  can  change  into  germplasm7  is, 
on  the  basis  of  my  conception,  deprived  of  any  founda- 
tion in  fact.  But  it  certainly  is  not  always  easy  to  decide 
whether  a  track  is  to  be  regarded  as  a  somatic  one  or  as 
a  germ-track,  as  will  be  seen  from  the  next  chapter. 

For  a  clear  comprehension  of  the  phenomena  of  he- 
redity the  conception  of  the  germ-tracks,  as  it  has  been 

4Weismann,  A.  Zur  Frage  nach  der  Unsterblichkeit  der  Einzellig- 
en.    Biolog.  Centr.  4:    683. 
5Loc.  cit.  p.  685.  , 

6 Weismann,  A.   Die  Kontinuitdt  des  Keimplasmas.  p.  11. 
7Loc.  cit.  p.  52. 


Germ-Tracks  and  Somatic  Tracks  91 

modified  above,  seems  to  me  to  be  of  prime  importance. 
While  natural  selection  appears  to  act  upon  the  qualities 
of  the  finished  organism,  in  reality  it  acts  upon  the  bearers 
of  these  characters  hidden  in  the  germ-cells.8  This  im- 
portant law  has  been  raised  above  all  doubt  by  the  ex- 
periences of  animal  and  plant-breeders.  Vilmorin,  in  his 
breeding  experiments,  distinguished  the  individuals  which 
possessed  in  a  higher  degree  the  power  of  transmitting 
their  visible  qualities  to  their  descendants  from  those  that 
possessed  it  to  a  lesser  degree.9  The  former  he  called 
bons  etalons,  and  those  he  selected  for  breeding.  But 
whether  a  plant  belonged  to  this  privileged  group  the  plant 
itself  did  not  show.  This  had  to  be  decided  by  the  de- 
scendants and  by  these  was  the  great  breeder  guided  in 
the  selection  of  his  breeding  plants. 

The  body  of  the  individual,  therefore,  gives  only  a 
one-sided  and  very  incomplete  indication  of  the  qualities 
represented  in  its  germ-tracks.  But  when  one  grows 
from  its  seeds  hundreds  and  thousands  of  specimens,  these 
furnish  such  a  many-sided  picture  that  the  average  may 
be  regarded  as  a  criterion  of  those  latent  attributes. 

By  far  the  most  of  the  hereditary  character-units  at- 
tain their  development  only  in  the  somatic  paths ;  it  is  only 
here  that  the  corresponding  characters  of  the  organism 
become  visible  to  us.  But  the  transmission  of  a  char- 
acter and  its  development  are,  as  Darwin  says,10  distinct 
powers  which  need  not  necessarily  run  parallel.  The 
transmission  is  accomplished  invisibly,  in  the  germ-tracks, 

8Weismann,  A.  Ueber  die  Vererbung.  p.  56. 

9Vilmorin,  L.  L.  de.  Notices  sur  I'  amelioration  des  plantes  par 
le  semis.  Nouvelle  Edition,  p.  44.  1886. 

10Darwin,  C.  The  Variation  of  Animals  and  Plants.  2:  38.  New 
York,  1900. 


92  Cell-Pedigrees 

the  development  mostly  on  the  somatic  tracks.  It  is  only 
with  caution  that  we  may  utilize  the  latter  in  judging  the 
former. 

In  the  following  chapter  I  will  discuss  more  in  detail 
the  germ-tracks  and  the  somatic  tracks  in  the  cell-pedi- 
gree of  the  higher  plants.  In  doing  so  I  shall  divide  the 
former  into  primary  and  secondary  germ-tracks.  Both 
lead  from  the  fertilized  egg-cell  to  the  new  egg-  or  sperm- 
cell.  The  former  ones,  however,  do  so  by  the  shortest 
route,  that  is  usually  within  one  individual,  and,  in  the 
case  of  alternation  of  generations,  through  the  usually 
small  number  of  individuals  involved.  The  latter,  on  the 
contrary,  reach  their  end  indirectly,  by  means  of  vegeta- 
tive multiplication,  e.  g.,  through  adventitious  buds.  They 
may  frequently  pass  through  an  apparently  unlimited 
number  of  individuals  before  returning  to  an  egg-cell. 


CHAPTER  II 
SPECIAL  CONSIDERATION  OF  THE  INDIVIDUAL  TRACKS 

§  5.  The  Primary  Germ-Tracks 
I  designate  as  primary  germ-tracks  those  sequences 
of  generations  of  cells  which,  in  the  normal  course  of 
development  of  the  organism,  lead  from  the  fertilized 
egg-cell  to  the  new  germ-cells  (egg-cells,  spermatozoa, 
pollen-grains).  They  will  form  the  subject  of  the  first 
paragraphs.  The  secondary  germ-tracks,  leading  through 
adventitious  buds,  will  be  considered  in  the  subsequent 
paragraphs. 

The  primary  germ-tracks,  then,  form  the  common, 
or  at  least  the  shortest  of  the  common,  paths  from  one  to 
the  next  following  generation  of  egg-cells.  They  are 
never  completely  unbranched,  because  the  normal  multi- 
plication of  the  species  is  incumbent  on  their  ramification. 
They  probably  always  give  off  somatic  twigs  along  their 
entire  length.  But  the  manner  and  means  of  their  ram- 
ification, the  number,  position,  and  relative  significance 
of  the  individual  somatic  tracks,  is  subject  to  much  modi- 
fication. 

Among  extreme  cases  may  be  counted  one  one  side 
the  well  known  instance  of  the  Diptera,  on  the  other  hand 
the  Vertebrates,  and,  contrasted  with  both,  the  higher 
plants  and  the  corals.  In  the  Diptera  some  of  the  first 
cells  that  usually  form  from  the  egg  develop  into  the  sex- 
ual glands  of  the  body.  Thus  the  initial  cells  for  prac- 
tically the  entire  body  are  directly  separated  from  the 


94  The  Individual  Tracks 

germ-track  at  the  first  divisions,  and  this  forms  thereafter, 
only  the  somatic  tracks  lying  in  the  sexual  glands.  To 
the  Diptera  must  be  added  the  Daphnoidae  and  Sagitta, 
for  the  whole  body  of  which,  with  the  exception  of  the 
organs  of  reproduction,  the  initial  cells  are  also  split  off 
very  early  from  the  germ-track,  and  by  means  of  a  rela- 
tively small  number  of  cell-divisions.  In  the  vertebrates 
the  germ-track  probably  goes  through  hundreds  of  suc- 
cessive cell-divisions,  for  the  purpose  of  body-formation, 
before  it  begins  the  development  of  the  sexual  organs. 

Leaving  the  sexual  organs  out  of  our  consideration,  we 
find  that  the  somatic  tracks  composing  the  body  arise 
from  the  germ-track,  in  the  Diptera  as  a  single  twig,  in 
the  Daphnoidae  and  Sagitta  as  a  small  number  of 
them,  in  the  vertebrates,  however,  as  very  numerous 
twigs.  But  all  the  tracks  for  the  body  are  always  formed 
before  the  germ-track  begins  to  split  into  equivalent 
branches  in  the  region  of  the  sexual  organs. 

Here  lies  the  difference  between  the  higher  animals 
and  the  plants.  For  in  the  latter  the  germ-track  splits 
at  a  very  early  period,  and  the  majority  of  the  somatic 
tracks  do  not  originate  in  the  main-stem  of  the  germ- 
track,  but  chiefly  in  its  branches.  The  picture  of  the  pedi- 
gree of  the  germ-cells  coincides  here  with  the  picture  of 
the  much  ramified  organism  itself;  it  does  not  require  a 
detailed  description.  The  colony-forming  polyps  present 
a  similar  case. 

The  difference  becomes  clearest  on  introducing  into 
the  picture  only  the  germ-tracks,  and  leaving  out  the  so- 
matic tracks.  The  cell-pedigree  of  a  higher  animal  stands, 
then,  as  a  straight  tree,  ramifying  only  a  little  at  its  top, 
while  that  of  the  higher  plants  is  so  richly  and  repeatedly 
branching  from  its  very  origin  that  the  branches  fre- 


Primary  and  Secondary  Germ-Tracks  95 

quently  overtop  the  main-stem  which  thus,  not  infre- 
quently/is in  the  back-ground  of  the  picture.  Or,  more 
correctly  speaking,  there  is  no  real  main-stem,  or  at  least 
hardly  any. 

§  6.     The  Secondary  Germ-Tracks 

In  the  higher  animals  the  secondary  germ-tracks  are 
lacking,  in  the  vegetable  world  they  are  widely  distrib- 
uted. It  is  especially  this  circumstance  which  makes  the 
study  of  cell-pedigrees  in  the  vegetable  kingdom  so  much 
more  profitable  than  in  the  animal  world,  and  the  objec- 
tions raised  by  Sachs,  Strasburger,  and  other  botanists 
against  Weismann's  conception  regard  essentially  the  cir- 
cumstance that  the  latter  did  not  give  due  attention  to 
the  secondary  germ-tracks. 

The  secondary  germ-tracks  can  by  no  means  be  re- 
garded as  exceptions.  In  no  tree,  in  no  shrub  are  they 
lacking.  Among  perennial  plants  they  are,  if  not  of  gen- 
eral occurrence,  at  least  very  widely  distributed,  and  only 
the  annual  and  biennial  species  are  without  this  kind  of 
propagation.  On  the  other  hand  the  adventitious  forma- 
tions exhibit  so  many  forms,  such  high  differentiations, 
and  such  beautiful  adaptations,  that  they  also  are  not 
placed  in  the  background,  in  this  respect,  as  compared 
with  the  primary  germ-tracks. 

For  our  purpose  three  cases  are  to  be  kept  separate: 

1.  Nearly  all  cells  of  the  body  can  develop  into  new 
individuals. 

2.  Adventitious  buds  arise  only  from  definite  cell- 
groups  or  cell-tracks  preformed  to  this  end,  namely : 

a.  from  meristematic  tissues, 

b.  from  mature  cells. 

The  phenomena  of  regeneration  of  the  Thallophyta 


96  The  Individual  Tracks 

and  the  Muscineae  have  in  recent  years  repeatedly  been 
the  subject  of  investigation,  and  the  conviction  has  be- 
come rooted  in  regard  to  them  that,  at  least  in  some  cases 
of  mutilation,  every,  or  almost  every  cell  that  remains 
unhurt  can  grow  into  a  new  individual.  Pringsheim  ex- 
amined the  mosses,  Vochting  the  liverworts,  Brefeld  the 
fungi.11  On  continuing,  under  favorable  conditions,  the 
cultivation  of  pieces  cut  off  from  these  plants,  one  can 
grow  a  new  plant  from  every  part  that  is  not  too  small. 
The  stipe  and  the  pileus  of  the  fungi  grow  new  pileuses 
from  the  cut  surfaces,  the  mosses  form  buds  from  any 
given  cell  of  the  roots,  leaves  and  shoot,  even  from  the 
sporangium  and  its  stalk.  At  first  the  cells  grow  into  the 
thread-like  protonema,  on  which  the  leaf -buds  can  then 
develop  in  the  usual  manner.  The  Marchantiaceae,  ac- 
cording to  Vochting,  can  be  chopped  up  fine,  and  every 
particle  which  has  a  sufficient  number  of  uninjured  cells 
to  keep  it  alive,  will  form  a  new  plant.  In  the  case  of 
Marchantia  polymorpha  I  can  confirm  this  observation 
from  my  own  experience. 

In  these  cases,  therefore,  all,  or  nearly  all  the  ramifi- 
cations of  the  cell-pedigree  form  either  primary,  or  at 
least  secondary  germ-tracks.  Somatic,  that  is,  necessar- 
ily sterile  twigs  are  possibly  present,  although  it  has  not 
yet  been  proven.  This  case,  which  for  Weismann  forms 
an  exception,  and  demands  a  special  assumption  for  its 
explanation,12  is  for  us  only  an  extreme  one  in  the  rich 
abundance  of  examples. 

11  Pringsheim,  N.  Ueber  Sprossung  der  Moosfruchte.  Jahrb. 
Wiss.  Bot.  11:  1.  1878. 

Brefeld,  O.  Botanische  Untersuchungen  iiber  Schimmelpilze, 
Vol.  I.  Vochting,  H.  Ueber  die  Regeneration  der  Marchantiaceen. 
Jahrb.  Wiss.  Bot.  16:  367.  1885. 

12 Weismann,  A.    Die  Kontinuitdt  des  Keimplasmas.  p.  68. 


Secondary  Germ-Tracks  97 

The  second  group  of  secondary  germ-tracks,  the  ad- 
ventitious buds  from  meristematic  tissues,  is  by  far  the 
most  widely  distributed  in  the  vegetative  world.  Adven- 
titious buds  arise  in  part  directly  from  the  normal  meri- 
stematic tissues,  in  part  throught  the  medium  of  the  cal- 
lus-tissue which  leads  to  the  closing  up  of  wounds. 
Those  that  originate  from  stems  or  branches,  usually 
become  new  twigs  of  the  individual  bearing  them,  the 
leaf-born  ones  and  the  root-buds,  however,  develop  for 
the  most  part  into  new  plantlets. 

Bud-formation  from  callus  is  chiefly  found  in  woody 
plants,  and  almost  every  part  of  a  branch  or  a  root,  if  cut 
for  a  slip  or  otherwise  injured,  can  develop  from  the 
youthful  cells  of  the  cambial  zone,  situated  between  the 
wood  and  the  bark,  that  undifferentiated  tissue,  oozing 
out  like  drops  of  a  semi-fluid  substance,  in  which  later 
cork,  bark,  and  wood,  as  well  as  the  rudiments  of  numer- 
ous buds  develop.  According  to  circumstances  the  buds 
become  roots  or  leafy  twigs,  and  usually  replace  the  lost 
members  of  the  individuals. 

Since,  as  far  as  we  know,  every  cell  of  the  cambium 
may  contribute  to  the  callus,  and  can  produce  therein  the 
mother-cell  of  a  bud,  we  must  designate  the  entire  cam- 
bium as  a  secondary  germ-track  which  is  as  profusely 
ramified  as  the  cell-pedigree  of  the  respective  cambium 
itself,  and  which  bears  the  normal  products  of  its  activity, 
wood  and  bark,  as  countless  somatic  twigs.  It  is  to  be  re- 
membered, however,  that  many  cells  of  the  wood  and  bark 
retain,  for  a  longer  or  shorter  time,  the  power  of  con- 
tributing to  the  formation  of  the  callus,  and  even  of  pro- 
ducing mother-cells  of  callus-buds.13  The  line  of  de- 
marcation between  the  secondary  germ-tracks  and  the 

13This   point   indeed   still   requires   thorough   investigation. 


98  The  Individual  Tracks 

somatic  tracks  is  therefore  to  a  great  extent,  obliterated 
here,  and  perhaps  even  quite  undemonstrable. 

Callus-buds  are  also  to  be  found  in  many  herbaceous 
plants.  On  leaves,  too,  they  are  not  rare,  but  in  such 
cases  they  usually  form  new  rooted  plantlets. 

Adventitious  buds  on  leaves  are  very  frequent  phe- 
nomena among  the  ferns.  In  the  phanerogams  they  arise 
at  the  base  of  detached  leaves,  especially  in  bulbous  plants 
and  Crassulacese.  Very  well  known  instances  are  fur- 
ther furnished  by  Bryophyllum  calycinum,  Cardamine  pra- 
tensis,  and  Nasturtium  officinale.™  There  can  be  no  doubt 
that  in  all  of  these  cases  there  is  present  in  every  leaf 
a  germ-track,  which  is  very  frequently  much  ramified. 

Root-buds  are  probably  the  most  common  and  cer- 
tainly the  most  completely  and  most  thoroughly  investi- 
gated adventitious  buds.15  And  since  many  leaves,  like 
slips  from  stems  and  roots,  can  form  roots  after  having 
been  detached  from  the  plant  and,  by  means  of  these 
roots,  give  life  to  new  plantlets,  the  importance  of  the  root- 
buds  can  hardly  be  exaggerated.  Many  plants,  such  as 
Monotropa,  multiply,  except  by  seed,  only  in  this  manner, 
others,  like  Rumex  Acetosella  and  the  thistles  become  the 
most  tenacious  weeds  by  means  of  root-buds.  Of  all  spe- 
cies that  possess  this  power,  we  can  therefore  say  that 
their  root-system  represents,  in  the  cell-pedigree,  a  much 
ramified  germ-track  with  its  somatic  twigs. 

14From  the  abundant  literature  on  this  subject  I  cite:  Regel, 
Vermehrung  der  Begonien  aus  ihren  Blattern.  Jenaische  Zeits. 
Naturw.  p.  478.  1876.  Beyerinck,  Over  het  ontstaan  van  knoppen  en 
wortels  uit  bladeren.  Ned.  Kruidk.  Archief.  3:  1.  1882.  Wakker,  J. 
H.  Ondersoekingen  over  adventieve  knoppen.  Amsterdam,  1885. 

15This  subject  has  been  most  exhaustively  treated  by  Dr.  M.  W. 
Beyerinck  in  his  "Beobachtungen  und  Betrachtungen  iiber  Wurzel- 
knospen  und  Nebenwurzeln."  Verhandl.  Kon.  Akad.  Wetenschappen. 
Amsterdam,  1886. 


Adventitious  Buds  99 

I  should  like  to  go  further  into  this  rich  and  tempting 
field.  But  the  reader  who  is  familiar  with  the  literature 
will  not  need  my  guidance  in  forming  a  picture  of  the 
secondary  germ-tracks  in  the  cell-pedigree,  and  in  arriv- 
ing at  the  conclusion  that  almost  every  larger  branch  of 
this  tree  is  to  be  regarded  as  a  germ-track. 

We  still  have  to  deal  with  the  third  case,  that  of  the 
adventitious  buds  from  mature  cells.  Here  the  secondary 
tracks  run  through  formed  cells,  which  frequently  begin 
only  in  an  advanced  age  to  rejuvenate,  and  to  grow  into 
buds.  This  is  illustrated  by  the  begonias,  which  Darwin 
has  already  used  in  his  pangenesis  for  the  explanation  of 
the  almost  universal  distribution  of  the  hereditary  char- 
acters throughout  all  the  parts  of  the  plant-body,16  and 
which  Sachs  and  Strasburger  considered  as  opposing 
Weismann's  theory  of  the  germ-plasm.  This  phenom- 
enon has  been  thoroughly  studied  by  Regel,  Beyerinck, 
and  Wakker,17  and  it  seems  sufficiently  important  to  me  to 
be  sketched  here  in  its  main  lines. 

The  epidermal  cells  of  the  leaves  and  petioles,  and  also, 
in  some  forms  (e.  g.,  Begonia  phyllomaniacaj  those  of 
the  stem  and  its  branches,  possess  the  power  of  becoming 
buds.  This  power  is  not  limited  to  individual,  privileged 
cells,  at  least  not  in  the  leaves,  but  is  inherent  to  the  same 
extent  in  all  cells  of  the  epidermis,  especially  in  those  of 
the  veins.  If  part  of  a  leaf  is  laid  on  the  ground  in  moist 
air,  after  the  veins  have  been  previously  cut  through  in 
several  places,  there  may  be  found,  after  some  time,  near 
each  wound,  one  or  several  new  plantlets.  The  first  pri- 
mordium  of  these  is  a  true  rejuvenation.  The  epidermal 

16Darwin,  C.  The  Variation  of  Animals  and  Plants.  2:  362. 
New  York.  1900. 

17See  citations  above  (p.  98). 


100  The  Individual  Tracks 

cell,  poor  in  contents,  divides,  without  at  first  gaining  in 
size,  into  a  small-celled  body  of  tissue,  in  which  rich  pro- 
toplasmic contents  can  now  be  observed.  Gradually  this 
new  formation  grows  and  differentiates,  by  means  of  nu- 
merous further  cell-divisions  into  a  bud. 

Since  these  germ-tracks,  which  lead  through  a  mature 
but  rejuvenating  cell  to  a  new  generation,  possess  a  high 
theoretical  value,  and  will  be  frequently  mentioned  in  the 
following  pages,  I  shall  give  them  a  new  name,  and  call 
them  fiseudosomatic. 

§  7.     The  Somatic  Tracks 

As  Nussbaum  has  so  strikingly  put  it,  the  germ  tracks 
are  "the  continuous  foundation  stock  of  the  species,  from 
which  the  single  individuals,  after  a  short  existence,  fall 
like  withered  leaves  from  a  tree."  With  the  difference 
that  every  leaf  is  attached  to  the  tree  at  some  point, 
whereas  most  individuals  consist  of  the  products  of  nu- 
merous somatic  tracks,  which  have  originated  successively 
from  the  germ-track,  and  therefore  cannot  fall  off  without 
a  piece  of  the  foundation  stock. 

The  somatic  tracks  composing  the  individual  usually 
differ  greatly  from  each  other.  Not  only  morphologi- 
cally, in  regard  to  the  kind  of  cells,  tissues,  and  organs  to 
which  they  lead,  but  also  in  their  size  and  the  extent  of 
their  ramification.  The  whole  aerial  plant  of  Equisetum, 
in  the  first  year  of  its  existence,  represents  a  somatic 
ramification.  The  leafy  twigs  of  Tax  odium,  which  fall 
off  in  the  autumn,  and  the  leaves  of  all  those  plants  which 
are  not  capable  of  reproducing  their  species  by  means  of 
those  organs,  are  further  illustrations.  There  is  an  unin- 
terrupted line  of  intermediate  steps  from  these  to  the  one- 


The  Somatic  Tracks  101 

celled  somatic  tracks  which  do  not  ramify  any  further,  as 
for  example,  the  wood-fibres  of  some  trees  which  are  pro- 
duced by  the  cambium. 

The  somatic  tracks  are,  in  general,  the  cell-pedigrees 
of  the  single  cells  of  the  grown  individual,  with  the  excep- 
tion of  the  germ-cells.  In  the  case  of  every  cell  and  every 
cell-complex  one  can  trace  them  back  to  the  germ-track 
from  which  they  have  evolved.  In  plants  all  the  profusely 
branching  primary  and  secondary  germ-tracks  are  prob- 
ably closely  set,  along  their  entire  length,  with  such  bushy 
lateral  twigs.  These  give  its  characteristic  appearance  to 
our  picture.  In  the  Diptera  they  originate  chiefly  from 
one  point  of  the  germ-track,  and  thereby  the  picture  is 
entirely  changed.  In  the  higher  animals,  however,  they 
gradually  branch  off  from  the  unramified  part  of  the 
germ-track,  and  very  greatly  surpass  it  in  the  richness  of 
their  further  ramifications. 

The  cells  of  the  somatic  tracks  are  usually  composed 
of  the  same  protoplasmic  organs  as  those  of  the  germ- 
tracks.  Only  here  these  organs  are  frequently  adapted 
to  other  functions,  and  therefore  they  bear  other  names. 
Thus,  in  some  somatic  elements,  the  amyloplasts  of  the 
germ-track  cells  become  chloro'phyll-grains.  Usually  this 
change  is  not  only  a  more  special  adaptation,  but  also  a 
further  differentiation.  Especially  do  we  meet  again,  al- 
most without  exception,  in  all  somatic  cells,  such  indi- 
vidual parts  of  the  germ-track  cells  as  nucleus,  tropho- 
plast,  vacuoles,  nucleo-plasm,  and  lining  layer. 

Against  this  general  rule  some  individual  exceptions 
must  be  mentioned.  I  do  not  take  into  account  the  nu- 
merous cells,  such  as  the  many  wood-fibres,  and  the  stone- 
cells  and  cork-cells,  which  die  soon  after  their  development 
and  lose  their  entire  protoplast.  They  render  their  ser- 


102  The  Individual  Tracks 

vices  to  the  organism  in  this  lifeless  condition,  and  form 
the  extreme  instance  of  a  reduction  on  the  somatic  tracks. 

But  there  are  also  cases  of  a  lesser  reduction.  Fre- 
quently, in  the  Algae,  as  Schmitz  describes,  "In  the  in- 
terior of  the  cells,  the  chromatophores,  of  which  there  is 
no  longer  any  need,  and  which,  in  the  economy  of  the 
whole  plant,  were  equipped  and  adapted  exclusively  for  a 
definite  single  function,  disappear."18  Especially  is  this 
often  the  case  in  complexly  organized  and  highly  differ- 
entiated algae.  Sometimes,  as  it  would  seem,  in  the  in- 
most tissue-cells,  but  most  commonly  in  the  hairs  and 
rhizoids. 

A  further  instructive  instance  is  given  by  the  spore- 
sacs  of  the  Ascomycetse.  In  these  flask-like  cells  there 
originate,  through  the  division  of  the  nucleus,  the  nuclei 
for  the  individual  spores,  while  the  mother-cell,  according 
to  the  available  data,  does  not  retain  any  nucleus.  When 
the  spores  are  formed  the  mother-cell  has,  therefore,  be- 
come a  non-nucleated  protoplast,  although  it  has  by  no 
means  completed  its  life-task,  since  it  has  still  to  take  a 
very  active  part  in  the  extruding  of  the  spores,  for  which 
purpose  it  must  retain,  in  the  interior  of  its  numerous 
vacuoles,  the  necessary  osmotic  pressure. 

In  our  cell-pedigrees  the  ripe  ascus  forms  the  last 
somatic  twig  of  the  germ-track  which  culminates  in  its 
spores.  This  twig  is  simple,  i.  e.,  it  does  not  necessarily 
branch  further.  What  lends  importance  to  this  illustra- 
tion, however,  is  the  present  conception  of  the  significance 
of  the  nucleus.  For,  if  it  is  the  seat  of  the  latent  hered- 
itary characters,  we  may  assume  that  these  are  lacking 
in  the  ripe  ascus.  And  evidently  the  latter  does  not  need 

18Schmitz,  Die  Chromatophoren  der  Algen.  p.  137.  1882. 


Difference  Between  Somatic  and  Germ-Tracks    103 

them  for  the  fulfillment  of  the  functions  still  devolving 
upon  it. 

Therefore,  we  have  here  an  instance  of  a  somatic  path 
without  latent  hereditary  qualities.  At  least,  this  is  as 
certain  as  observation  can  make  it  in  the  present  state  of 
our  knowledge.  And  it  is  evident  that  this  instance  com- 
pels the  assumption  that  on  many  other  somatic  tracks,  as 
well,  a  reduction  of  the  hereditary  characters,  although 
less  extensive,  may  take  place.  But  since  our  task  is  to 
group  facts,  and  not  to  make  assumptions,  we  shall  not 
discuss  this  point  any  further. 

§  8.  The  Difference  Between  Somatic  Tracks  and  Germ- 
Tracks 

We  see  now  before  us  the  rough  lines  of  the  picture 
of  the  cell-pedigrees  for  the  higher  plants.  And  whoever 
followed  my  description  attentively,  will  have  seen  that 
the  picture  is  a  purely  empirical  one,  in  which  the  promi- 
nent lines  are  indeed  arbitrarily  chosen,  but  have  been 
drawn  without  any  hypothesis.  Especially  is  the  differ- 
ence between  the  somatic  and  the  germ-tracks  purely  a 
matter  of  fact,  and  in  harmony  with  our  present  knowl- 
edge. It  claims  nothing  except  to  serve  as  an  indication  as 
to  whether  any  cell  can,  through  its  descendents,  con- 
tribute to  the  propagation  of  the  species. 

But,  as  a  basis  for  theoretical  considerations,  the  cell- 
pedigrees  will  attain  their  full  value  only  when  we  have 
realized  the  significance  of  the  difference  between  somatic 
and  germ-tracks.  This  is  by  no  means  a  difference  in 
kind,  but  one  of  degree.19  This  becomes  clearest  to  us 
when  we  try  to  define  the  limit  exactly.  We  shall  find, 

19Weismann,  A.  Zur  Annahme  einer  Kontinuitat  des  Keim- 
plasmas.  Ber.  Naturf.  Ges.  Freiburg.  1:  7.  1886. 


104  The  Individual  Tracks 

then,  that  an  apparently  uninterrupted  line  of  transitional 
forms  leads  from  the  germ-tracks  to  the  somatic  tracks. 

In  the  cell-pedigrees  of  one-celled  organisms  and  of 
homoplastids  all  the  twigs  are  primary  germ-tracks.  In 
the  next  higher  plants  primary  and  secondary  germ- 
tracks  are  to  be  distinguished  and,  the  more  highly  the 
organism  is  differentiated,  the  more  are  the  latter  pushed 
into  the  background.  They  are  lacking  in  the  higher  ani- 
mals. But  in  such  highly  developed  Thallophytes  as  the 
fungi,  and  even  in  the  mosses  and  liverworts,  it  is  ap- 
parent that  all  twigs  in  our  picture  have  still  the  value  of 
germ-tracks.  At  least  sterile  side-twigs,  that  is,  somatic 
tracks,  have  not  yet  been  demonstrated  there.  But,  in  the 
case  of  the  vascular  plants,  most  of  the  tissue-cells,  at 
least  when  fully  developed,  can  without  doubt  no  longer 
reproduce  the  species.  Therefore  the  somatic  tracks  form 
here  an  important  part  of  the  picture. 

But  let  us  now  compare  the  somatic  tracks  of  the  vas- 
cular plants  with  the  secondary  germ-tracks  of  the  Mus- 
cineae.  Were  not  the  significance  of  the  latter  known  to 
us  through  the  investigations  of  Pringsheim  and  Voch- 
ting,  we  would  designate  at  least  some  of  them  as  so- 
matic tracks,  for  the  question  can  be  decided  only  by  the 
presence  or  absence  of  the  power  of  reproduction.  On 
the  other  hand,  it  may  possibly  be  shown,  at  some  future 
time,  that  some  somatic  cells  of  the  vascular  plants  have 
this  power  after  all,  and  what  we  now  call  somatic  tracks, 
we  will  then  have  to  regard  as  secondary  germ-tracks. 

The  somatic  tracks  have  obviously  developed  phyloge- 
netically  from  the  secondary  germ-tracks.  Not  suddenly, 
however,  and  at  a  leap,  but  quite  gradually.  The  loss  of 
the  power  of  reproduction  makes  them  such.  By  this 
means,  however,  only  an  adaptation,  and  no  intrinsic  dif- 


Difference  Between  Somatic  and  Germ-Tracks    105 

ference  is  conferred.  It  is  true  that,  through  further 
adaptions,  the  differences  may  have  become  greater  and 
greater;  the  use  of  the  power  of  reproduction,  at  first 
limited  to  less  and  less  frequent  cases,  may  finally  have 
become  quite  impossible  by  the  loss,  not  only  of  the  adapt- 
ive, but  also  of  the  inner  conditions  thereto.  Doubtless 
all  transitions  to  the  non-nucleated  spore-sacs  will  have 
been  made. 

But,  in  the  plant  world,  by  far  the  greatest  number 
of  the  somatic  tracks  are  evidently  still  so  much  like  the 
secondary  germ-tracks  that  we  cannot  assume  an  essential 
difference  between  them.  This  is  most  clearly  demon- 
strated in  those  cases  where  homologous  organs  among 
allied  species  consist,  in  one  of  them,  of  somatic  tracks 
only,  while  the  other  possesses  secondary  germ-tracks  in 
addition. 

The  most  instructive  illustration  is  given  in  the  pseudo- 
somatic  germ-tracks  of  the  begonias.20  Phylogenetically 
these  have  obviously  originated  from  tracks  that  we  should 
call  somatic.  But  the  very  circumstance  that,  in  the  pro- 
cess of  the  formation  of  species,  this  power  of  reproduc- 
tion can  make  its  appearance  in  cells  in  which  it  is  lacking 
in  almost  all  the  other  phanerogams,  teaches  us  that  this 
absence  is  only  adaptive,  I  might  almost  say  only  apparent. 
We  are  therefore  compelled  to  attribute  to  the  epidermal 
cells  of  the  leaves  of  the  phanerogams  in  general  a  latent 
power  of  reproduction.  Yet  they  remain  recorded  as 
somatic  tracks  in  our  empirical  picture.  Nevertheless  it 
seems  perfectly  clear  to  me  that  the  difference  is  not  quali- 
tative. 

Furthermore,  the  correctness  of  this  conception  is  cor- 
roborated by  the  not  at  all  infrequent  instances  where 

2°Cf.  p.  100. 


106  The  Individual  Tracks 

parts  of  plants,  which  normally  cannot  form  buds,  produce 
such  in  accidental  variations  or  in  varieties.  Flower-bear- 
ing twigs  have  been  observed  on  a  petal  of  a  Clarkia  and 
of  a  Begonia,  on  the  stem  of  the  compound  leaf  of  Lyco- 
persicum,  and  on  the  leaves  of  Levisticum,  Siegesbeckia, 
Rheum,  Urtica,  and  Chelidonium.  Caspary  saw  more 
than  a  hundred  of  them  on  a  petiole  of  Cucumis.  Every- 
one is  doubtless  familiar  with  the  flowers  on  the  glumes 
of  the  variety  of  barley  cultivated  as  Hordeum  trifurca- 
tum. 

Some  leaves  can  take  root  when  cut  off  and  stuck  into 
moist  ground.  I  saw  those  of  Aucuba  and  of  Hoya  car- 
nosa  keep  alive,  in  this  way,  for  two  years,  without  form- 
ing buds;  some  are  said  to  have  existed  for  seven  years 
in  this  condition.21  Whether  buds  are  ever  developed  from 
the  roots  of  such  leaves,  either  normally  or  after  wound- 
ing, seems  to  be  unknown.  But  this  is  not  at  all  impossi- 
ble, and  in  general  the  whole  case  deserves  to  be  more 
thoroughly  investigated.  Other  leaves  fail  to  take  root 
under  like  conditions,  and  simply  perish.  But  those  of 
the  Crassulaceae,  and  of  bulbous  plants,  grow  buds  from 
their  base.  Here,  too,  the  line  of  demarcation  between 
somatic  tracks  and  secondary  germ-tracks  is  evidently  not 
a  sharp  one,  at  any  rate  not  qualitative. 

Finally,  we  have  still  to  emphasize  the  fact  that  very 
frequently  the  power  of  reproduction  is  restricted  to 
youth.  This  is  most  clearly  shown  by  the  callus-forma- 
tion of  woody  plants,  where  the  still  living  older  cells  of 
the  bark  and  the  wood  usually  do  not  take  any  part  in  it. 
In  the  petioles  of  plants  that  are  rich  in  juice,  as  Peper- 

21I  have  since  succeeded  in  keeping  a  rooted  leaf  of  Hoya  car- 
nosa  alive  for  more  than  six  years.  It  did  not  produce  any  bud.  de  V. 
1909. 


Phyletic,  Somar  tar  chic  and  Somatic  Cell-Divisions  107 

omia,  grown  cells  also  take  part  in  the  callus- formation, 
but,  as  it  seems,  only  in  a  subordinate  way.  Perhaps  by 
far  the  greatest  part  of  the  somatic  cells  of  plants  have 
this  power  in  their  youth,  and  the  line  of  demarcation 
between  secondary  germ-tracks  and  somatic  tracks  would 
lose  still  more  of  its  distinctness  through  this  possibility. 

§  p.     Phyletic,  Somatarchic,  and  Somatic  Cell-Divisions 

We  will  now  look  a  little  more  closely  into  the  cells 
themselves,  which  are  distributed  along  the  individual 
tracks.  In  the  homoplastids  all  the  cells  and  all  the  cell- 
divisions  have  the  same  importance.  The  two  daughter- 
cells  evolved  from  one  mother-cell  are  of  the  same  value. 

But  in  the  higher  plants  such  processes  are  relatively 
rare.  They  happen  chiefly  only  where  a  germ-track  di- 
vides into  two  equivalent  branches,  or  where  a  uniform 
tissue  is  deposited  on  a  somatic  track.  By  far  the  greatest 
number  of  divisions,  however,  furnish  unlike  products, 
and  to  this  fact  is  due  the  entire  differentiation. 

It  seems  more  important  to  me  to  distinguish  between 
phyletic,  somatarchic,  and  somatic  cell-divisions.  Those 
divisions  in  which  a  germ-track-cell  splits  into  two 
daughter-cells,  both  of  which,  although  in  different  ways, 
continue  the  germ-track,  are  obviously  phyletic.  All  the 
somatic  cell-divisions  are  divisions  on  the  somatic  tracks. 
Where  a  track  is  laid  down  of  such  a  nature  that  through 
the  division  of  a  cell  of  the  germ-track,  there  develops,  on 
the  one  hand,  a  cell  which  continues  the  germ-track,  and 
on  the  other  hand,  a  somatic  cell,  the  division  is  soma- 
tarchic. 

There  can  be  no  doubt  that,  in  the  phyletic  divisions, 
the  hereditary  factors  are  transmitted  to  the  two  daughter- 


108  The  Individual  Tracks 

cells.  Such  is  the  case,  also,  in  the  somatarchic  divisions, 
with  reference  to  the  daughter  cells  that  continue  the 
germ-track.  But  as  to  whether  or  not  this  also  holds  true 
of  the  other  sister-cell,  which  forms  the  beginning  of  a 
somatic  track,  opinions  differ.  As  to  whether  or  not,  in 
the  somatic  cell-divisions,  a  corresponding  reduction  of 
the  latent  factors  goes  hand  in  hand  with  the  advancing 
adaptation  and  specialization  of  the  cells  will  be  discussed 
in  the  next  chapter. 

I  have  still  to  emphasize  that  the  successive  genera- 
tions of  cells  from  the  germ-tracks,  which  evolve  from  so- 
matarchic cell-divisions,  are  not  all  alike.  They  have  been 
designated  at  times  either  as  germ-cells  or  as  embryonic 
cells.  But  there  is  no  necessary  reason  for  this  in  the  plant 
kingdom.  It  is  true  that  they  are  all  alike  in  being  the 
bearers  of  all  the  hereditary  characters  of  the  species,  but 
they  bear  them  only  in  a  latent  condition.  They  may  be  in- 
trinsically very  different  in  respect  to  their  active  heredi- 
tary characters.  And,  even  if  the  whole  germ-track  does 
not  pass  through  such  a  rich  variety  of  forms  and  adapta- 
tions as  are  furnished  to  us  by  the  somatic  cells,  yet,  com- 
pared with  a  single  somatic  path,  however  profusely  the 
latter  may  branch,  it  may,  by  no  means,  be  second  to  the 
latter  in  regard  to  differentiation.  On  the  contrary,  the 
very  power  of  producing,  one  after  another,  the  most 
varied  somatic  tracks,  indicates  a  continuous  alteration  in 
its  activity. 

The  cells  of  the  germ-tracks  are  by  no  means  always 
such  as  remain  in  a  juvenile  condition  during  the  whole 
duration  of  their  existence,  or  which,  between  quickly  suc- 
ceeding cell-divisions,  have  only  a  short  individual  life. 
The  prothallia  of  ferns  and  horse-tails  consist  of  green, 
vigorously  assimilating  cells,  through  the  divisions  of 


Transmission  vs.  Development  of  Characters     109 

which  there  is,  at  first,  an  increase  in  number,  until,  at 
last,  from  some  of  them  the  sexual  organs  develop.  There- 
fore the  cells  on  the  main  germ-tracks  are  here  not  dis- 
tinguished by  any  visible  characteristic  from  the  purely 
vegetative  cells.  The  same  is  true  of  the  already  repeatedly 
mentioned  pseudo-somatic  germ-tracks  of  the  begonia. 

Everywhere  we  are  confronted  with  the  statement  of 
Darwin,  quoted  above,  that  the  transmission  and  the  de- 
velopment of  hereditary  characters  are  different  powers. 
In  the  cell-pedigree  they  run  almost  nowhere  parallel. 


CHAPTER  III 
WEISMANN'S  THEORY  OF  THE  GERM-PLASM 

§  jo.  The  Significance  of  the  Cell-Pedigree  for  the  Doc- 
trine of  the  Germ-Plasm 

In  the  first  two  chapters  of  this  section  I  have  compre- 
hensively described  the  cell-pedigrees  for  the  plant  world, 
and,  in  order  to  draw  a  clear  picture,  I  have  been  com- 
pelled to  introduce  a  number  of  new  names.  The  fact  that 
all  the  cells  of  the  whole  plant-body  are  produced  by 
division,  is  now  universally  recognized,  and  herewith  the 
possibility  of  the  establishment  of  cell-pedigrees  is  admit- 
ted as  a  matter  of  course.  Furthermore,  the  scientific 
value  of  such  consideration  has  been  pointed  out  by  dif- 
ferent investigators  in  botany  as  well  as  in  zoology. 

The  elaboration  of  the  picture,  however,  as  I  men- 
tioned in  the  beginning  of  this  division  of  Part  II,  seemed 
indispensable  to  me,  because,  up  to  the  present  time,  the 
higher  animals  have  been  put  to  the  front  in  these  consid- 
erations, and  for  the  further  reason  that  this  fact  leads 
only  too  readily  to  a  one-sided  conception.  For  here  the 
distinction  between  the  germ-cells  and  the  body-elements 
is  so  great  that  it  only  too  easily  gives  the  impression  of  a 
qualitative  difference. 

This  contrast  has  been  strongly  emphasized  by  Weis- 
mann  in  his  interesting  speculations  on  the  "mortal"  so- 
matic cells  and  the  "immortal"  germ-cells,22  and  forms,  to 
a  large  extent,  the  basis  for  his  theory  of  the  germ-plasm. 

22Weismann,  A.  Ueber  die  Dauer  des  Lebens.  1882.  Ueber  Leben 
und  Tod.  1884. 


Theory  of  the  Germ-Plasm  111 

This  doctrine,  and  the  hypothesis  of  the  ancestral 
plasms  which  is  based  on  it,  have  already  been  critically 
reviewed  in  the  first  Part.  I  have  there  (p.  56)  also 
pointed  out  the  fact  that,  in  the  face  of  a  detailed  consid- 
eration of  cell-pedigrees,  it  cannot  be  maintained.  Now 
that  we  have  become  more  familiar  with  these  latter,  it 
must  be  our  task  to  endeavor  to  establish  this  claim. 

The  true  significance  of  the  difference  between  the 
germ-tracks  and  the  somatic  cells  can  be  correctly  judged 
only  when  glancing  over  the  whole  richness  of  the  ramifi- 
cations of  a  highly  differentiated  cell-pedigree.  And  it  is 
only  in  plants  that  this  differentiation  reaches  its  highest 
degree.  Numerous  intermediate  forms  lead  here,  with 
almost  imperceptible  transitions,  from  the  main  germ- 
track  to  the  somatic  tracks. 

For  this  very  reason  I  have  laid  particular  stress  on 
the  discussion  of  the  secondary  germ-tracks.  They  are 
wanting  in  the  higher  animals.  In  the  plant  kingdom  they 
are  present  in  all  gradations.  I  have  not  attempted  to 
draw  a  sharp  line  of  demarcation  between  them  and  the 
main  germ-tracks ;  such  an  attempt  would  be  thwarted  by 
the  same  difficulties  which  make  impossible  the  exact  lim- 
itation of  the  concept  "individual."  We  must  be  satisfied 
here  with  an  arbitrary  limit,  and  choose  the  one  that  seems 
most  convenient. 

The  difficulties  that  confront  us  on  the  border-line  be- 
tween secondary  germ-tracks  and  somatic  tracks  are  of 
a  different  nature.  Here  they  are  due  to  the  incomplete- 
ness of  our  knowledge.  I  call  those  tracks  that  do  not 
lead  to  a  propagation  of  the  species  somatic.  But  many 
cells,  many  a  tissue-complex  which,  on  this  ground,  we 
now  call  somatic,  will  prove  itself,  on  further  experimen- 
tation, to  be  provided  with  the  power  of  reproduction. 


112  Theory  of  the  Germ-Plasm 

The  group  of  the  pseudo-somatic  tracks  may  be  chosen  as 
an  illustration,23  and  I  shall  come  back  to  further  instances 
in  the  last  paragraph  of  this  Section. 

Therefore  germ-cells  and  somatic  cells  do  not  present 
any  qualitative  contrast  in  the  plant  kingdom.  They  are  the 
extremes  of  a  long  line  of  quantitative  differences.  This 
law  I  regard  as  one  of  the  most  important  results  of  the 
consideration  of  vegetative  cell-pedigrees.  Sachs,  Stras- 
burger,  and  others,  have  pointed  out  the  importance  of 
this  law,  and  it  seems  to  me  that  the  foregoing  compre- 
hensive descriptions  ought  to  contribute  in  causing  the 
conviction  of  its  correctness  to  become  general. 

On  the  distinction  between  germ-cells  and  somatic 
cells  Weismann  founded  his  theory  of  the  germ-plasm. 
The  latter  must,  therefore,  be  present  in  all  the  germ-cells. 
But  according  to  Weismann,  it  is  only  in  these  that  it  needs 
to  be  retained,  while  it  must  be  lacking  in  the  somatic 
cells,  because  they  cannot  reproduce  the  species.  They 
are  limited  to  the  unfolding  of  a  limited  number  of  hered- 
itary units,  and  thus  need  only  that  portion  of  the  germ- 
plasm  requisite  thereto.  These  considerations  induced 
Weismann  to  regard  the  germ-plasm  as  a  special  sub- 
stance, which,  in  contrast  to  the  remaining  or  somatic 
plasm,  is  the  vehicle  of  heredity. 

In  the  first  part  we  have  seen  how  the  theory  of  a  germ- 
plasm  fails  us  in  the  explanation  of  the  differentiation  of 
organs.  There  the  assumption  of  one  substance  is  not 
sufficient ;  special  material  bearers  of  the  individual  hered- 
itary characters,  the  so-called  pangens,  were  necessary  for 
the  explanation.  Their  assumption,  however,  rendered 
the  assumption  of  the  germ-plasm  with  its  consequences, 
superfluous. 

23Cf.     Section  6.  p.  100. 


The  Views  of  Botanists  113 

Now  we  have  demonstrated  that  the  empirical  basis 
for  the  assumption  of  the  germ-plasm,  which  was  to  lie  in 
the  qualitative  difference  between  germ  and  somatic  cells, 
was  only  an  apparent  one  and  disappears  when  we  con- 
sider cell-pedigrees  in  detail,  and  from  every  point  of 
view. 

Nor  from  this  point  of  view  can  we  recognize  as  justi- 
fied the  assumption  of  the  germ-plasm.  Because  if  we 
were  to  attribute  germ-plasm  to  all  the  cells  of  the  en- 
tire organism,  the  hypothesis  would  thereby  become 
superfluous,  and  the  term  practically  synonymous  with  nu- 
cleo-plasm. 

I  propose  to  follow  out  these  general  discussions  more 
in  detail  in  the  two  following  subdivisions  of  this  chapter. 

§  ii.     The  Views  of  Botanists 

That  all  the  cells  of  the  germ-tracks  must  contain 
the  hereditary  characters  of  their  species,  in  either  the 
active  or  the  latent  state,  can  hardly  be  doubted.  How  the 
somatic  cells  behave  in  this  respect,  cannot  on  the  whole 
be  determined  by  experiment.  Especially  not  negatively, 
because  the  absence  of  latent  hereditary  characters  can 
never  be  experimentally  proven.  The  quite  isolated,  non- 
nucleated  cells  of  nucleated  .organisms  form  possibly  an 
exception.  But  positive  experimental  results  would  lead 
us  to  recognize  the  investigated  cells,  which,  up  to  that 
time  had  been  called  somatic,  as  elements  of  secondary 
germ-tracks.  Therefore  they  only  shift  the  limit  without 
deciding  the  question. 

And  yet,  as  we  have  seen  in  the  preceding  paragraph, 
the  question  is  one  of  high  theoretical  value.  And  as 
long  as  this  point  has  at  all  been  an  object  for  reflection, 
botanists  have  been  of  the  opinion  that  all,  or  at  least  by 


114  Theory  of  the  Germ-Plasm 

far  the  most,  of  the  cells  of  the  plant-body  have  been 
equally  endowed  in  regard  to  latent  characters.  Turpin 
and  Schwann,  later  Miiller  and  Hanstein,  but  in  recent 
years,  especially  Vochting,  have  taken  up  the  pen  in  the 
support  and  development  of  this  view. 

This  prevailing  and  so  well  substantiated  doctrine  was 
opposed  by  Weismann  in  the  year  1885.  He  advanced  his 
well  known  theory  of  the  continuity  of  the  germ-plasm, 
and  thus  sought  to  create  a  basis  for  a  theory  of  heredity. 
The  material  bearer  of  the  hereditary  characters  in 
their  totality,  and  including  therefore  the  latent  ones, 
Weismann  calls  germ-plasm ;  the  bearers  of  the  active 
qualities  in  any  given  cell,  somatic  plasm.  The  somatic 
plasm  is,  therefore,  lacking  in  no  cell,  because  they  are  all 
active  to  a  certain  degree,  even  if  only  to  the  extent  of 
being  capable  of  further  division.  The  germ-plasm,  how- 
ever, is,  according  to  him,  restricted  to  those  cells  which 
are  charged  with  the  transmission  of  the  hereditary  char- 
acters to  the  following  generations.  In  the  true  somatic 
cells  this  power  is  said  to  be  lacking. 

Intimately  connected  with  this  conception,  according 
to  Weismann,  is  the  law  that  the  character  of  every  cell 
is  determined  by  its  nucleus.24  The  specific  nature  of  a  cell, 
according  to  him,  is  dependent  on  the  molecular  structure 
of  its  nucleus;  every  histologically  differentiated  kind  of 
cell  possesses  therefore  its  specific  nucleo-plasm.25  Identi- 
cal nucleo-plasm,  ceteris  paribus,  means  also  identical  cell- 
body  ;  in  every  somatarchic  cell-division,  as  well  as  in  most 
of  the  somatic  divisions,  the  nucleo-plasm  must  therefore 
split  into  two  unequal  parts,  only  that  part  of  the  hered- 
itary characters  being  given  to  each  daughter-cell,  which 

24E.  g.  Die  Kontinuitat  des  Keimplasmas.  p.  30. 
25Loc.  cit.  p.  70. 


Objections  to  the  Theory  115 

is  necessary  for  the  functions  of  its  descendents.26  If  the 
progeny  be  unlimited,  as  in  the  germ-tracks,  then  the  nu- 
cleus receives  the  entire  germ-plasm;  but  since  the  pro- 
geny of  a  somatarchic  cell  is  limited,  and  since  it  is 
restricted  in  its  morphological  and  physiological  range  of 
development,  it  gets  only  the  corresponding  part  of  the 
hereditary  characters.  Therefore  they  have  no  true  germ- 
plasm,  but  only  somatic  plasm. 

On  the  hypothesis  of  the  germ-plasm,  Weismann 
builds  that  of  the  ancestral  plasm,  which  is  directly  op- 
posed to  pangenesis,  and  has  been  critically  considered 
in  the  last  division  of  Part  I.  But  the  empirical  justifica- 
tion for  the  basis  of  that  assumption,  may  here  be  con- 
sidered from  every  possible  point  of  view. 

That  Weismann  has  not  succeeded  in  convincing  bot- 
anists is  shown  by  the  various  objections  to  him,  made 
especially  by  Sachs  and  Strasburger.  The  essence  of  these 
objections  is  that  Weismann  has  not  sufficiently  consid- 
ered the  secondary  germ-tracks,  and  has  thus  been  in- 
duced to  assume  a  sharp  contrast  between  germ-plasm  and 
somatic  plasm.  Now,  not  only  the  oft  mentioned  exam- 
ple of  the  begonias,  but  the  entire  and  very  rich  doctrine 
of  adventitious  buds,  teach  that  there  is  nowhere  a  sharp 
line  of  demarcation  between  the  secondary  germ-tracks 
and  the  somatic  tracks  of  the  plant.  The  latter  have  de- 
veloped only  quite  gradually  out  of  the  former.  And 
even  though  they  have  in  fact  often  lost  the  power  of  re- 
production, everything  speaks  in  favor  of  the  fact  that 
they  still  very  frequently  possess  it  potentially.  In  other 
words,  the  loss  of  germ-plasm  need  not  necessarily  go 
hand  in  hand  with  the  loss  of  the  power  of  reproduction. 

In  his  book,  Ueber  Organbildung  im  Pflanzenreich, 

28Cf.  also  Part  I,  Chapter  III,  §  6,  p.  S3. 


116  Theory  of  the  Germ-Plasm 

published  about  ten  years  ago,27  Vochting  brought  to- 
gether the  facts  known  at  that  time  and  the  results  of  his 
own  rich  experiments.  At  the  end  of  the  first  volume  he 
discusses  the  pending  question  in  detail.  The  experiments 
teach  directly  (p.  251),  that  "in  every  fragment,  be  it 
ever  so  small,  of  the  organs  of  the  plant-body,  rest  the 
elements  from  which,  by  isolating  the  fragment,  under 
proper  external  conditions,  the  whole  body  can  be  built 
up."  Of  course,  this  is  true  only  if  the  fragment  contains  a 
number  of  meristematic  cells.  On  this  basis  the  question  is 
discussed,  "Whether  there  is  a  sufficient  support  for  ex- 
tending our  proposition  over  any  given  complex  of  living 
vegetative  cells."  This  discussion  again  leads  to  the  as- 
sumption that  every  morphological  form  of  tissue  is  po- 
tentially in  a  condition  to  produce  meristematic  cells,  and 
therefore  to  reproduce  the  entire  oVganism.  But  since 
experiments  involving  the  isolation  of  very  small  portions 
of  tissues  encounter  unsurmountable  difficulties,  and  since, 
on  the  other  hand,  the  power  of  reproduction  as  an  adap- 
tation may  very  likely  have  been  lost  in  many  tissues, 
there  is,  as  a  matter  of  course,  no  "strict  proof  attempted, 
and  it  is  simply  claimed  that  this  very  plausible  assump- 
tion is  probably  correct."28 

This  assumption,  however,  in  the  now  current  lan- 
guage, has  no  other  meaning  than  that  all,  or  at  least  the 
greatest  number  of  the  cells  of  the  plant-body  contain  all 
the  hereditary  characters  of  the  species  in  a  latent  condi- 
tion. And  this  same  assumption  I  have  sought  to  estab- 
lish, as  far  as  possible,  empirically,  through  a  detailed 
description  of  cell-pedigrees  available  through  the  most 
recent  investigations  on  the  phenomena  of  regeneration. 

"Vol.-  1,'Bonn,  1878;  Vol.  II,  Bonn,  1884. 
.  cit.  pp.  251-253. 


Objections  to  the  Theory  117 

It  is,  indeed,  not  to  be  denied  that  Weismann's  view 
finds  strong  theoretical  support  in  the  usual  economy  of 
nature.  Why  endow  numberless  cells  and  long  genera- 
tions of  cells  with  characters  which  they  will  never  need  ? 
But  it  must  not  be  forgotten  that  such  parsimony  would 
perhaps  necessitate  special  adaptations,  and  that  therefore 
it  might,  in  the  end,  be  simpler  not  to  make  any  differ- 
ences at  all  between  the  individual  cells  in  regard  to  their 
latent  characters. 

However,  I  should  not  like  to  go  quite  so  far  as  to  at- 
tribute to  every  somatic  cell  all  the  latent  qualities.  First 
of  all,  as  was  pointed  out  at  the  beginning  of  this  Part, 
it  would  be  impossible  to  support  such  a  view  experiment- 
ally, and  therefore  it  would  remain  permanently  sterile. 
Then  I  have  pointed  out  the  non-nucleated  asci,  which 
doubtless  represent  somatic  tracks  without  latent  hered- 
itary units,  and  therefore  permit  the  assumption  of  a  re- 
duction of  these  qualities  in  other  tracks.  Here,  too,  a 
very  slowly  advancing  differentiation  and  specialization 
is,  on  the  whole,  much  more  probable,  according  to  our 
present  conception  of  living  nature,  than  the  sharp  con- 
trast between  the  chosen  bearers  of  heredity  and  the  so- 
matic cells  equipped  only  with  the  hereditary  particles 
required  for  their  functions,  as  assumed  by  Weismann. 

Weismann  also  expresses  himself,  on  the  ground  of 
botanical  facts,  to  the  effect  "that  he  can  see  no  theoreti- 
cal obstacle  to  the  germ-plasm,  under  certain  conditions, 
being  admixed  with  cells  of  a  pronounced  histological  dif- 
ferentiation, or,  indeed,  even  with  all  the  cells  of  the  en- 
tire plant/'  For  the  liverwort,  serving  as  a,n  illustration, 
he  admits  this  conclusion  to  be  correct.29  And  the  more 

29Zur  Annahme  einer  Kontimiitat  des  Keimplasmas.  Ber.  Nat- 
urforsch.  Ges.  Freiburg.  1:  10.  1886. 


118  Theory  of  the  Germ-Plasm 

we  study  the  cell-pedigrees  of  the  plant  kingdam,  the  more 
we  become  convinced  that  there  is  no  qualitative  distinc- 
tion in  nature  between  the  cells  of  the  germ-track  and  the 
somatic  cells. 

§  12.     A  Decision  Reached  Through  the  Study  of  Galls 

In  the  foregoing  paragraphs  we  have  repeatedly  em- 
phasized how,  on  the  whole,  it  is  impossible  to  decide  the 
pending  question  experimentally.  The  phenomena  of  re- 
production by  excised  parts  of  plants  make  manifest  the 
existence  of  secondary  germ-tracks  hitherto  unknown; 
but  they  do  not  teach  us  anything  about  the  nature  of  the 
remaining  somatic  tracks. 

An  experiment  which  we  cannot  carry  through  is  made 
by  the  gall-forming  parasites  in  such  a  great  variety  of 
ways  that  a  glance  at  their  products  may  be  made  at  this 
point.  The  thorough  and  detailed  examinations  by  Bey- 
erinck  have  so  far  enriched  our  knowledge  in  this  field, 
that  the  whole  history  of  development,  as  well  as  the  an- 
atomical structure  in  the  grown  condition,  is  clearly  laid 
before  us  in  the  case  of  all  the  more  important  forms  of 
galls.30  Two  laws,  especially  important  for  our  purpose, 
have  resulted  from  these  studies.  First  of  all,  the  galls, 
even  at  their  highest  differentiation,  are  built  up  of  only 
such  anatomical  elements  as  are  otherwise  found  in  the 
plant  bearing  them.  Only  the  peculiar  layer  of  stone  cells 
of  some  Cynipid-galls,  which  later  change  into  a  thin- 
walled  nutritive  tissue,  forms  a  hitherto  unexplained,  but 

80Beyerinck,  M.  W.  Beobachtungen  iiber  die  ersten  Entwick- 
elungsphasen  einiger  Cynipidengallen.  Veroffentlicht  Kais.  Akad. 
Wiss.  Amsterdam.  1882.  The  same,  Die  Galle  von  Cecidomia  Poae. 
Bot.  Zeit.  43:  305,  321.  1885,  and  Ueber  das  Cecidium  von  Nematus 
capreae.  Bot.  Zeit.  46:  1.  1888. 


Importance  of  the  Study  of  Galls  119 

probably  only  apparent,  exception  from  this  rule.  In  the 
second  place  plants  have  no  special  adaptations  for  the 
purpose  of  gall-formation ;  the  adaptations  lie  completely 
with  the  parasite  which  works  only  with  the  characters 
that  belong  to  its  host. 

But  the  galls  are  not  at  all  restricted  to  the  anatomical 
elements  of  the  organs  on  which  they  originate.  Cells 
which  the  plant  otherwise  forms  in  the  bark  of  its  stem 
only,  can  frequently  be  found  in  the  galls  of  leaf-inhabit- 
ing Cynipids  and  Diptera.  The  same  holds  true  for  the 
galls  of  the  stem  and  the  root.  We  may  conclude  from 
this  that  the  power  of  producing  these  elements  belongs 
not  only  to  those  organs  which  develop  them  normally, 
but  probably  also  to  all  the  other  parts  of  the  plant. 

Worthy  of  special  notice  here  are  the  roots  which,  for 
the  purpose  of  covering  the  galls  of  Cecidomia  Poae,  de- 
velop in  a  place,  where,  in  the  normal  course  of  develop- 
ment, neither  the  plant  bearing  them,  Poa  nemoralis,  nor 
any  other  kind  of  grass,  is  able  to  produce  roots.31  Thus 
the  larvae  here  make  use  of  a  potentiality,  the  existence  of 
which  we  could  never  have  conjectured,  still  less  proven. 
In  Beyerinck's  experiments,  these  gall-roots  grew  into  nor- 
mal, profusely  ramifying  roots;  the  cells  of  the  internode, 
stimulated  to  activity,  must  therefore  have  possessed,  in 
a  latent  condition,  the  qualities  necessary  thereto. 

Through  the  experiments  of  this  investigator,  even  a 
direct  transformation  of  apparently  somatic  tracks  into 
germ-tracks  has  been,  if  not  entirely  accomplished,  at 
least  brought  quite  near  completion.82  The  galls  which  the 
leaf -wasp  Nematus  viminalis,  produces  on  the  leaves  of 
Salix  purpurca,  possess  an  exceeding  vitality.  At  the  be- 

^Bot.  Zeit.  1888.  1.  c. 

32Bot.  Zeit.  46:   1,  17.     1888. 


120  Theory  of  the  Germ-Plasm 

ginning  of  autumn,  when  left  by  their  inhabitants,  they 
are  still  quite  turgescent.  If  they  are  now  buried  in  hu- 
mus, they  will  keep  through  the  winter,  and  can  even 
enter  upon  a  new  life  in  the  following  summer.  They  will 
then  form  new  chlorophyll,  by  means  of  which  they  are 
nourished,  and  the  best  among  them  will  gradually  begin 
to  put  forth  adventitious  roots.  These  originate  either  on 
the  outer  or  on  the  inner  surface  of  the  wall  surrounding 
the  cavity,  and  are  always  located  on  the  vascular  bundles 
of  the  gall.  Judging  from  their  microscopic  structure, 
these  rootlets,  reaching  a  length  of  a  few  centimeters,  are 
identical  with  the  normal  young  roots  of  the  respective 
species  of  willow.  The  required  hereditary  characters 
must  therefore  be  present  in  a  latent  state  in  the  gall,  in 
which  probably  nobody  would  otherwise  have  looked 
for  a  germ-track. 

These  important  experiments  will  become  still  more 
instructive  for  our  purpose,  when  we  shall  succeed  in  mak- 
ing the  gall-roots  develop  so  far  that  they  are  enabled  to 
form  adventitious  buds.  But,  since  the  roots  of  all  woody 
plants  have  this  power,  we  may  predict  even  now  that  this 
experiment  will  succeed.  Perhaps  it  will  require  special 
measures,  as  for  example,  a  graft  on  the  roots  of  a  willow. 
But  without  doubt  we  may  conclude  from  the  complete 
agreement  in  the  anatomical  structure,  as  proven  by  Bey- 
erinck,  that  the  physiological  properties  also,  of  the  nor- 
mal and  of  the  gall-roots  are  the  same. 

And  if  anyone  is  ever  successful  in  growing  in  this 
way  an  entire  willow  from  a  gall,  it  will  be  clear,  that,  in 
the  latter,  all  the  hereditary  characters  of  the  willow  are 
present  in  a  latent  state. 

This  would  obviously  be  much  more  useless  than  their 
presence  on  any  given  normal  somatic  track.  The  con- 


Importance  of  the  Study  of  Galls  121 

elusion,  however,  that  germ-plasm  is  by  no  means  limited 
to  those  cells  which  need  it  for  their  own  development,  nor 
to  their  progeny,  we  may  even  now  regard  as  perfectly 
certain. 

And  this  is  probably  the  most  important  inference 
which  we  may  deduce  from  this  entire  section.  With  it 
we  have  established  one  of  those  laws  which  can  be  ap- 
plied as  bases  for  our  hypothesis.  But  we  shall  revert  to 
this  in  the  last  Section. 


B.    PANMERISTIC  CELL-DIVISION 


CHAPTER  I 
THE  ORGANIZATION  OF  PROTOPLASTS 

§  i.     The  Visible  Organization 

Protoplasm  is  the  vehicle  of  the  phenomena  of  life, 
and  therefore  also  of  hereditary  characters.  Hence,  any 
theory  of  heredity  must  start  from  a  definite  view  in  re- 
gard to  the  structure  of  this  important  substance.  But 
anatomical  investigation,  in  spite  of  its  astonishing  prog- 
ress during  the  last  decade,  has  in  this  very  field  not  yet 
achieved  a  clear  and  generally  accepted  conception  of  this 
structure. 

This  is  essentially  due  to  the  circumstance  that  the 
newer  methods  for  the  study  of  the  nucleus  and  its  division 
have  disclosed  a  field  so  important,  and  so  rich  in  surpris- 
ing results,  that  attention  has  been  directed  chiefly,  and 
frequently  exclusively,  to  this  organ.  Often  one  even 
meets  with  views  which  put  the  protoplasm  (cytoplasm) 
into  the  background  with  reference  to  the  nucleus. 

But  the  study  of  the  nucleus  is  so  much  advanced  at 
present  that  one  may  hesitate  at  this  one-sided  treatment. 
The  researches  of  Flemming,  Strasburger,  and  so  many 
other  investigators,  have  disclosed  the  structure  of  the 
nucleus  and  the  changes  of  this  structure  during  division, 
and  have,  in  the  main,  brought  our  knowledge  to  a  definite 
conclusion.  Now,  especially  in  botany,  the  investigation 
of  cell-division  itself  comes  again  to  the  front.  And  it  is 
not  only  a  question  of  establishing  the  relation  of  the  nu- 
cleus to  the  cytoplasm ;  it  is  just  as  essential  a  problem  to 


126  The  Organisation  of  Protoplasts 

find  out  the  attitude  of  the  individual  organs  of  the  latter 
and  especially  of  the  vacuoles,  the  granular  plasm,  and  the 
plasmatic  membrane.  For  the  knowledge  of  cell-division 
will  be  complete  only  when  all  the  organs  of  the  proto- 
plast have  been  equally  considered. 

The  described  course  of  investigation  makes  it  clear 
why  even  a  practical  and  simple  designation  of  the  living 
cell-contents  has  not  yet  gained  general  recognition.  Such 
a  designation  was  suggested  by  Hanstein,  in  his  well- 
known  lectures,  by  the  word  "protoplast."1  The  word 
"protoplasm"  was  coined  by  Mohl  for  the  semi-liquid 
nitrogenous  substance  "which  furnishes  the  material  for 
the  formation  of  the  nucleus  and  the  primordial  utricle," 
and  from  which  originate  the  first  solid  structures  of  the 
future  cell.2  The  formed  body,  built  up  from  this  sub- 
stance, has  frequently  been  called  protoplasmic  body, 
plasm-body,  sometimes  even  protoplasmic  globule  or  drop, 
expressions  which  are  obviously  inadequate  to  create  a 
clear  conception  in  the  minds  of  readers  and  hearers. 

Compared  with  these  designations,  Hanstein's  word 
clearly  and  distinctly  describes  the  individuality  of  the 
living  cell-contents.   This  individuality  has  long  been  re~ 
ognized  by  the  best  investigators.     As  early  as    1862 
Brikke  said  that  protoplasm  was  an  organic  body;  not  " 
drop  of  fluid,  but  an  elementary  organism.8    But  the  lack 
of  an  appropriate  name  obscured  the  clearness  of  the  con 
ception,  and  it  was  Hanstein  who  supplied  this  want 
Klebs   and   others   have   accepted   his    designation   and 

1Hanstein,  J.  von.  Das  Protoplasma  als  Tr'dger  der  pflanzlichen 
und  thierischen  Lebensverrichtungen.  1  Theil.  1880. 

2Mohl,  H.  von.  Bot.  Zeit.  4:  75.    1846. 

3Briicke,  E.  Die  Elementaroganismen.  Sitsungsber.  Kais. 
Akad.  Wiss.  Wien.  442;  381.  1861. 


Protoplasts  Are  Elementary  Organisms        127 

through  their  influence  it  will  doubtless  be  more  gener- 
ally adopted.* 

Protoplasts  are  elementary  organisms  in  the  true  sense 
of  the  word.  They  consist  distinctly  of  individual  organs, 
which  are  more  or  less  sharply  distinguished  from  each 
other  and  which  possess  a  high  degree  of  mutual  indepen- 
dence. In  the  greatest  number  of  plants  this  structure  is 
clearly  evident,  but  in  the  lowest  organisms  this  differ- 
entiation is  entirely  wanting,  or  at  least  it  is  limited  to 
a  great  extent.  Sometimes  one  meets  with  the  expres- 
sion "unorganised  plasm,"  even  for  organisms  which  by 
no  means  lack  differentiation.  But  doubtless  this  expres- 
sion must  be  understood  to  mean  that  the  methods  so 
far  employed  have  not  yet  revealed  any  insight  into  the 
organization,  and  not  that  the  want  of  any  kind  of  organs 
has  been  thoroughly  studied  and  definitely  proven. 

*As  is  well  known,  the  term  is  now  in  common  use.     Tr. 


CHAPTER  II 
HISTORICAL  AND  CRITICAL  CONSIDERATIONS 

§  2.  The  Neo  genetic  and  the  Panmeristic  Conceptions  of 
Cell-Division 

Only  a  few  decades  back  it  was  generally  believed 
that  individual  organs,  such  as  the  nucleus  and  the  chlo- 
rophyll grains,  could  always,  or  at  least  very  frequently 
originate  from  the  undifferentiated  protoplasm  through 
differentiation.  However,  in  recent  years,  investigations 
have  not  confirmed  this  neogenesis  in  a  single  instance. 
Wherever  the  origin  of  an  organ  has  been  thoroughly 
and  comprehensively  studied,  with  the  present  means  of 
investigation,  the  organ  has  been  shown  to  originate 
by  a  division  of  differentiated  members  already  present. 

The  organization  of  the  protoplasts  is  not  periodical, 
nor  evident  only  in  grown  cells.  It  is  permanent,  inher- 
ent in  all  cells,  and  in  all  stages  of  their  development. 
The  assumption  of  formation  de  novo  gives  place  every- 
where to  the  recognition  of  divisions ;  the  neogenetic  con- 
ception gives  way  to  the  panmeristic.4 

It  is  of  interest  to  glance  over  the  course  of  develop- 
ment of  our  knowledge.  In  his  "Lehre  von  der  Pflan- 
zenzelle,"  Hofmeister  describes  the  nuclei  according  to 
the  knowledge  of  that  time.  They  appear  in  the  proto- 
plasm as  drops  or  masses  of  a  transparent  homogenous 
substance,  either  in  cells  with  few  nuclei,  of  a  definite 

4The  view  that  all  the  organs  of  protoplasts,  as  a  rule,  multiply 
only  by  division  I  call  panmeristic.  This  assumption  was  maintained 
for  plant-cells  for  the  first  time  in  my  plasmolytic  studies.  Cf.  Vries, 
H.  de.  Jahrb.  Wiss.  Bot.  16:  489.  1885. 


Neogenetic  vs.  Panmeristic  Cell-Division         129 

size  from  the  beginning,  or  in  cells  with  many  nuclei,  first 
as  small  formations  which  increase  through  growth. 
Sometimes  they  contain  granules  as  soon  as  they  become 
visible,  but  frequently  they  occur  at  first  without  any  inter- 
nal solid  structure,  and  attain  this  only  later.  Every  cell- 
division  is  usually  preceded  by  a  disappearance  of  the 
nucleus,  which  is  then  followed  by  the  appearance  of  two 
or  more  new  nuclei.5 

The  comprehensive  investigations  of  Strasburger  and 
Schmitz  have  proven  this  assumption  to  be  erroneous,  at 
first  for  isolated  and  then  for  an  increasing  number  of 
cases,  and  wherever  a  disappearance  and  subsequent  re- 
appearance of  nuclei  was  assumed,  the  origination  of  the 
new  nuclei  through  division  of  the  original  ones  could  be 
proven.  Exceptions  to  this  rule  are  no  longer  known. 

The  history  is  exactly  the  same  for  chlorophyll  grains. 
Even  in  the  last  edition  of  his  text-book6  Sachs 
said :  "The  chlorophyll  bodies  originate  in  young  cells 
through  the  separation  of  the  protoplasm  into  clearly 
distinct  colorless  portions  that  are  becoming  green. 
The  process  can  be  conceived  to  mean  that,  in  the 
originally  homogenous  protoplasm,  most  minute  particles 
of  a  somewhat  different  nature  are  distributed  or  origi- 
nate for  the  first  time  and  then  accumulate  at  various 
points,  appearing  as  differentiated  bodies."  That  the 
green  bodies  which  had  formed  in  this  way  could  multi- 
ply through  division,  and  that  the  chlorophyll  bodies  of 
many  algae  are  usually  cut  through  at  every  cell-division 
by  the  forming  wall,  can  easily  be  observed  and  was  not 
unknown  at  that  time. 

But  it  was  Schmitz  who  first  showed  that,  in  the  algae, 

BHofmeister,  Die  Lehre  von  der  Pflanzenzelle.  p.  79.  1887. 
*Lehrbuch  der  Botanik.  4.  Auflage,  p.  46.  1874. 


130          Historical  and  Critical  Considerations 

division  is  the  only  way  in  which  the  chromatophores  are 
newly  formed.7  Following  up  this  idea  with  the  phanero- 
gams, Schimper  discovered  the  colorless  organs  of  the 
youthful  cells,  which  in  these  cells  are  exclusively  charged 
with  the  formation  of  starch  and  through  whose  assump- 
tion of  green  color  the  real  chlorophyll  grains  are  formed. 
In  all  cases  that  have  been  observed  those  amyloplasts 
multiply  only  through  division,  and  Schimper,  as  well  as 
Arthur  Meyer,  has  accumulated  such  a  number  of  obser- 
vations on  this  manner  of  development  that  the  former 
view  has  been  abandoned  by  all  botanists.  Some  special 
cases,  it  is  true,  still  await  explanation,  but  as  long  as 
they  have  not  been  thoroughly  investigated,  there  is  no 
reason  for  regarding  the  old  conception  more  plausible 
than  the  new  one. 

It  is  similar  with  reference  to  the  vacuoles.  Until 
about  four  years  ago  they  were  generally  regarded  as  a 
new  formation  in  the  protoplasm,  caused  by  the  secre- 
tion of  superfluous  water  of  imbibition.  In  my  "Plas- 
molytische  Studien  ilber  die  Wand  der  Vacuolen,"  I  have 
established  the  claim  that,  for  them  as  well,  the  mode 
of  origin  of  nucleus  and  trophoplast8  must  be  the  only 
real  one.9  I  supported  this  claim  by  showing  that  all 
vacuoles  are  surrounded  by  a  living  wall,  which,  accord- 
ing to  the  method  suggested  by  me,  can  always  be  easily 
and  convincingly  demonstrated,  and  which  I  believe  may 
be  regarded  as  an  organ  of  the  protoplast,  with  as  much 
right  as  the  nuclei  and  the  chromatophores. 

This  conclusion,  drawn  from  my  panmeristic  concep- 
tion of  cell-division,  has  been  completely  confirmed  by 

7Schmitz,  F.  Die  Chromatophoren  der  Algen.  Bonn,  1882. 

8By  this  name  Arthur  Meyer  designates  the  amyloplasts  and 
their  derivatives  (chlorophyll  grains,  chromoplasts,  etc.) 

QJahrb.  Wiss.  Bot.  16:  489-505.    1885. 


Neogenetic  vs.  Panmeristic  Cell-Division         131 

Went's  investigations.10  Thereby,  to  my  mind  is  proven 
the  correctness  of  this  conception  as  opposed  to  that  of 
neogenesis.  Now  the  situation  is  reversed.  While  up 
to  the  present  time  the  condition  with  reference  to  the 
nucleus  and  the  chromatophores  could  be  regarded  as 
peculiar,  there  is  now  great  probability  that  the  different 
members  of  a  protoplast  have  the  same  mode  of  origin, 
and  therefore  that  they  can  claim  the  rank  of  independent 
organs  only  in  so  far  as  they  follow  this  rule. 

Now  that  the  mode  of  origin  for  nucleus,  trophoplasts 
and  vacuoles  has,  in  the  main,  been  established,  and  that 
the  works  of  Wakker11  have  taught  us  to  recognize  the 
crystals,  most  of  the  crystalloids,  and  the  aleurone  grains 
as  contents  of  the  vacuoles,  the  problem  is  chiefly  con- 
cerned with  the  plasmatic  membrane  and  the  granular 
plasm.12  In  regard  to  their  behavior  during  cell-forma- 
tion our  knowledge  is  essentially  the  same  as  at  the  time 
of  Mohl  and  Hofmeister.  Our  insight  into  the  pro- 
cess of  cell-division  has  indeed  become  deeper,  chiefly 
through  Strasburger's  work;  but  the  very  point  in  ques- 
tion, the  beginning  of  the  dividing  wall,  which  for  some 
time,  seemed  to  be  decided  neogenetically,  has  again  be- 
come extremely  uncertain  through  the  discovery  (to  be 
discussed  later)  of  the  cell-ring  by  Went13  as  well  as 
through  the  objections  of  other  investigators. 

10Went,  F.  A.  F.  C.  De  jongste  toestanden  der  vacuolen.  Amster- 
dam, 1886.  Les  premiers  etats  des  vacuoles.  Arch.  Neerl  1887,  and 
Die  Vermehrung  der  normalen  Vacuolen  durch  Theilung.  Jahrb. 
Wiss.  Bot.  19:  295.  1888. 

"Wakker,  J.  H.,  Studien  iiber  die  Inhaltskorper  der  Pflanzen- 
zellen.  Jahrb.  Wiss.  Bot.  19:  423.  1888.  Preliminary  contributions 
are  found  in  Maandblad  v.  Natuurwetensch.  1886,  Nr.  7.  1887,  Nr. 
5  and  6,  and  in  Bot.  Cent.  33:  360,  361.  1888. 

12Cf.     §  6  below,    p.  150. 

13Cf.   §  7  and  8,  pp.  157  and  160. 


132          Historical  and  Critical  Considerations 

For  these  reasons  I  believe  that  a  critical  review  of  our 
knowledge  in  this  field  will  be  of  substantial  usefulness. 
It  will  then  be  shown  how,  in  almost  all  cases,  the  attitude 
of  the  plasmatic  membrane  and  of  the  granular  plasm, 
during  cell-formation,  is  in  fact  unknown.  At  least  in 
all  the  cases  which  seem  to  contradict  the  panmeristic  con- 
ception. 

It  is  not  a  question  of  whether  this  latter  conception 
is  correct  or  not.  This  seems  to  me  to  have  been  proven 
above  any  doubt  by  the  researches  of  the  investigators  that 
have  been  quoted.  The  question  is  whether,  with  this 
conception,  we  are  to  regard  the  granular  plasm  and  the 
limiting  membrane  as  two  intrinsically  different  organs, 
which  pass  over  into  one  another  as  little  as  the  nuclueus 
and  the  chromatophores,  or  whether  they  stand  in  a  sim- 
ilar relation  to  each  other  as  the  amyloplasts  and  the  chlo- 
rophyll-grains. As  long  as  it  was  thought  that  the  gran- 
ular plasm  had  the  power  of  producing  the  other  members 
by  a  process  of  differentiation,  it  was  natural  to  assume 
a  like  mode  of  origin  for  the  plasmatic  membrance.  It 
is  therefore  not  astonishing  that,  even  at  present,  this 
view  is  still  regarded  as  the  one  that  actually  obtains. 
The  instance  described  by  Mohl  as  a  type  of  cell-di- 
vision, and  which  involved  the  historically  noteworthy 
discussions  of  the  question  as  to  whether  the  ^protoplasmic 
body  played  a  passive  or  an  active  role  during  this  process 
is  well  known  to  all.  Like  Mohl's  type  of  the  filamentous 
algae,  Cladophora,  Spirogyra  is  in  more  recent  times  pre- 
ferred for  this  study.  At  the  future  plane  of  division 
the  limiting  membrane  and  granular  plasm  fold  into  a 
ring  which,  growing  inwards,  apparently  simply  cuts  in 
two  the  remaining  part  of  the  cell-contents.  For  the 
daughter-cells  the  two  new  parts  of  the  limiting  membrane 


Autonomy  of  the  Plasmatic  Membrane          133 

originate  as  a  continuation  of  the  old  membrane.  Accord- 
ing to  Klebs's14  descriptions  the  Euglenidae  also  offer  a 
beautiful  example  of  panmeristic  cell-division. 

It  is  very  unlikely  that  in  the  case  of  such  a  funda- 
mental process,  the  higher  plants  should  behave  differ- 
ently from  the  lower  ones.  That  there  are  differences  in 
minor  points  is  self  evident,  and  everybody  knows  that 
there  are  important  distinctions,  especially  in  the  relative 
duration  of  the  individual  steps  in  the  process.  And  the 
same  holds  for  the  manner  in  which  it  is  provided  that 
every  daughter-cell  gets  its  own  nucleus.  But,  that  the 
completion  of  the  plasmatic  membrane  should  take  place 
through  the  insertion  of  a  quite  newly  formed  piece  is  so 
much  at  variance  with  the  rest  of  our  knowledge,  that 
one  cannot  by  any  means  accept  it  on  the  basis  of  the 
older  investigations.  At  any  rate  it  must  be  held  in  doubt 
until  supported  by  direct  observation. 

Such,  however,  is  not  the  case  at  present,  as  I  shall 
try  to  show  in  the  last  Chapter  of  this  Section.  On  the 
contrary  many  facts  already  speak  in  favor  of  the  com- 
plete autonomy  of  the  membrane,  although  not  with  suf- 
ficient certainty  to  serve  as  conclusive  proof. 

However  that  may  be,  whether  the  limiting  membrane 
can  develop  from  the  granular  plasm,  or  whether  both 
are  mutually  autonomous,  it  is  certain,  at  any  rate,  that 
on  the  one  hand  these  two,  and  on  the  other  the  nucleus, 
the  trophoplasts,  and  the  vacuoles  are  independent  organs, 
which,  in  the  normal  course  of  things,  multiply  only  by 
division. 

Hence,  the  organization  of  the  protoplasts  is  hered- 
itary, and  this  not  in  the  sense  that  the  organization  of  the 
higher  organisms  is  reproduced  in  each  individual  through 

14Klebs,  G.    Arbelten  Bot.  Institut.  Tubingen.  1:    282. 


134          Historical  and  Critical  Considerations 

the  development  of  invisible  hereditary  units,  but  through 
the  direct  passage,  from  the  mother-cell  to  the  daughter- 
cells,  of  all  the  organs  which  compose  the  organism. 

The  significance  of  this  law  for  our  hypothesis  of 
intracellular  pangenesis  will  be  discussed  in  the  last  divis- 
ion of  this  Part.  Here  we  will  familiarize  ourselves  more 
thoroughly  with  the  actual  basis  on  which  it  is  founded. 

§  j.     Cell-Division  According  to  Mohl's  Type 

The  "Grundzuge  der  Anatomie  und  Physiologie  der 
Vegetabilischen  Zelle,"  by  Hugo  von  Mohl,15  was  for  a 
long  time  the  chief  source  from  which  beginning  bota- 
nists got  their  knowledge.  It  is  only  Hofmeister's  Pflan- 
zenzelle  (1867)  and  Sachs's  Lehrbuch  (1868)  which  put 
an  end  to  its  reign,  but  many  illustrations  and  statements 
from  the  "Grundzuge"  are  still  vividly  remembered  by 
older  botanists. 

The  multiplication  of  cells  through  division  is  de- 
scribed in  the  following  manner  in  this  book  of  Mohl's.16 
It  "is  introduced  by  changes  which  the  primordial  utricle 
of  the  dividing  cell  undergoes,  in  consequence  of  which 
the  dividing  walls  develop,  growing  gradually  inward 
from  the  periphery  of  the  cell,  and  separating  the  cell- 
cavity  into  two  or  more  cavities."  We  have  to  dis- 
tinguish those  cases  where  the  cell-division  is  preceded 
by  a  doubling  of  the  nucleus,  from  those  in  which  this 
is  not  the  case  (our  present  poly-nucleate  cells).  This 
latter,  less  frequent,  but  simpler  case  occurs  in  Conferva 
glomerata,  and  therefore  Mohl  begins  his  description 
with  this  alga.  But  even  where  the  formation  of  two 
new  nuclei  precedes  the  formation  of  the  dividing  wall, 

15Published  in  Wagner's  Handworterbuch  der  Physiologie,  1851. 
16Loc.  cit.  p.  211. 


Cell-Division  According  to  Mohl's  Type         135 

this  latter  process  takes  place  in  the  same  manner  as  in 
the  Conferva  above  mentioned.  And  this  as  well  among 
the  algae  as  in  the  higher  plants.  According  to  Mohl, 
then,  the  plasmatic  membrane  is  always  produced  by  new 
parts  growing  out  of  old  ones. 

As  to  the  historical  aspect,  it  needs  only  to  be  em- 
phasized that  this  law  for  the  algae,  which  Mohl  put 
into  the  foreground,  has  been  confirmed  by  all  later  in- 
vestigators.17 Here  its  correctness  is  beyond  any  doubt, 
and  can  be  easily  controlled  by  anybody.  Who,  therefore, 
on  theoretical  grounds,  is  inclined,  to  assume  that,  in  cell- 
division,  the  same  principles  are  valid  for  the  entire  plant- 
world,  must  with  Mohl,  still  regard  the  case  in  question 
as  a  type. 

In  the  uni-nucleate  cells  there  are  usually  present  very 
peculiar  structures,  the  function  of  which  is  to  make  the 
new  dividing  wall  pass  exactly  between  the  two  new 
nuclei.  From  our  present  conception  of  the  significance 
of  the  nucleus  this  cannot  be  wondered  at,  for  what  would 
a  cell  be  without  its  hereditary  characters.  In  the  higher 
plants  these  structures  are  not  cleared  up  in  every  respect, 
though  with  the  spirogyras  this  is,  to  a  large  extent,  the 
case,  especially  through  the  repeated  publications  of 
Strasburger.  We  shall  therefore  describe  the  process 
in  this  plant,  making  use  of  the  last  description  of  this 
investigator  as  far  as  this  serves  for  our  purpose. 

At  the  time18  when  the  nucleus  approaches  the  end 
of  the  prophase,  the  protoplasm  collects  around  it  and 

17Cell-division  through  constrictions  is  widely  distributed  among 
the  lower  algae.  Cf.,  e.  g.,  Klebs,  Arbeiten  Bot.  Inst.  Tubingen.  1: 
336-343. 

18The  following  is  taken  from  Strasburger,  Ueber  Kern-  und 
Zclltheilung  im  Pflanzcnrdch.  pp.  9-23.  Jena,  1888. 


136          Historical  and  Critical  Considerations 

assumes,  in  the  region  of  the  poles  of  the  nucleus,  a  struc- 
ture of  parallel  fibres.  It  soon  becomes  clear  that  we  have 
to  do  with  the  first  signs  of  the  spindle-fibres.  These 
develop  quickly  and  continue  through  the  interior  of  the 
nuclear  cavity,  until  they  come  into  contact  with  each 
other.  There  is  no  valid  reason  for  the  eventual  assump- 
tion that  the  spindle-fibres  developing  in  the  interior  of 
this  cavity  are  of  a  different  origin  from  the  external 
ones.  On  the  aequator  of  the  spindle  the  chromatic  sub- 
stance accumulates,  touching  the  individual  fibres  at  their 
circumference. 

Next  occurs  the  formation  and  longitudinal  splitting 
of  the  nuclear  skein,  followed  by  the  separation  and 
moving  apart  of  the  two  halves  of  the  segments.  Dur- 
ing this  period  one  sees  clearly  that  not  all  the  spindle- 
fibres  have  succeeded  in  uniting  with  the  opposite  ones. 
Only  those  that  were  successful  in  this  are  retained  as 
connecting  fibres  between  the  two  young  nuclei  which 
move  apart.  The  space  forming  between  them  is  sur- 
rounded by  a  protoplasmic  mantle  toward  the  outside, 
and  apparently  there  collects  in  it  a  substance  with  osmotic 
action  which  enlarges  this  space  and  drives  the  young 
nuclei  apart.  In  the  meantime  the  number  of  the  con- 
necting fibres  on  the  mantle  of  this  space  is  lessened  more 
and  more,  the  mantle  itself  is  made  to  bulge  more  and 
more  in  a  transverse  direction,  and  becomes  correspond- 
ingly thinner.  Yet  it  remains  sharply  and  plainly  visible. 
The  space  has  assumed  now  the  well-known  barrel-shape, 
its  wall  is  called  the  connecting  cylinder,  and  remains  for 
some  time  as  an  extended  vesicle,  closed  in  on  all  sides. 
Finally,  by  being  strongly  distended  in  an  aequatorial 
direction,  this  vesicle  reaches  the  protoplasmic  accumu- 
lation at  the  margin  of  the  protruding  dividing  wall.  It 


Cell-Division  According  to  Mohl's  Type         137 

unites  with  the  latter,  and  is  now  gradually  flattened  by 
it,  and  finally  constricted. 

According  to  the  principles  of  the  theory  of  the  vacu- 
oles  ascertained  by  Went  and  myself,  it  is  probable  that 
the  space  containing  osmotic  substance  and  surrounded 
by  the  connecting  cylinder  is  a  vacuole,  which,  contrary 
to  Strasburger's  conception,19  must  have  penetrated  from 
the  outside  between  the  two  younger  nuclei.  It  is  just  as 
evident  that  this  vacuole  must  be  surrounded  by  a  wall 
of  its  own,  and  that  this  also  forms  the  inner  layer  of  the 
connecting  cylinder.  The  latter  is  also  separated  from 
the  other  vacuoles  of  the  cell-space,  by  a  wall,  and  between 
the  two  walls  there  lies,  at  least  in  the  beginning,  granu- 
lar plasm.  The  changes  of  that  vacuole  which  forms  the 
interior  of  the  barrel  during  the  whole  process  require, 
of  course,  special  investigation,  made  on  living  material.20 

But  there  can  be  no  doubt  about  the  correctness  of 
Strasburger's  conception,  where  he  places  the  whole  pro- 
cess of  cell-division,  with  the  one  exception  of  the  divi- 
sion of  the  nucleus,  in  the  protoplasm  itself.  The  daugh- 
ter-nuclei are  passive  in  this,  the  cytoplasm  alone  is  the 
active  element. 

The  chlorophyll-bands,  the  vacuole,  and  the  granular 
plasm  are  simply  constricted  by  the  plasmatic  membrane 
growing  into  the  interior.  The  membrane  itself  finally 
separates  in  the  same  manner,  after  having  entirely  closed 
up  the  space  remaining  in  the  middle  of  the  ring. 

In  those  poly-nucleate  algae,  the  nuclei  of  which  are 
evenly  distributed  over  the  entire  lining  layer  of  proto- 

l9Loc.  dt.  p.  17. 

20Zacharias,  in  his  discussion  of  Strasburger's  work  (Bot.  Zeit. 
46:  449.  1888),  emphasizes  also  "that,  on  the  living  object,  things  may 
exist  which  can  be  better  recognized  and  interpreted  there  than  by 
fixing  and  staining." 


138          Historical  and  Critical  Considerations 

plasm,  no  particular  devices  have  been  observed  for  as- 
suring the  possession  of  one  or  more  nuclei  at  the  cell- 
divisions  of  each  daughter-cell.  Moreover  they  do  not 
seem  necessary,  owing  to  the  great  number  and  regular 
distribution  of  the  nuclei.  Nuclear  spindle  and  nuclear 
barrel  have  therefore  lost  their  significance  in  this  case, 
and  accordingly  they  are  probably  not  present,  at  least 
not  as  a  rule.  Cell-division  is  essentially  performed  by 
the  plasmatic  membrane  and  the  granular  plasm  only. 

For  the  correct  understanding  of  the  processes  of 
normal  cell-division,  one  law,  which  has  been  ascertained 
by  experiments  on  artificial  division  of  living  protoplasts 
in  former  and  more  recent  times,  is  of  extreme  import- 
ance. I  do  not  mean  the  adaptive  processes  of  regener- 
ation after  wounding  (these  will  be  discussed  in  the 
next  paragraph),  but  the  constriction  of  the  uninjured 
cell-contents  in  entire  cells,  and  the  division  of  the  pro- 
toplasts into  two  or  more  pieces  during  plasmolysis.  The 
respective  cases  I  have  put  together  in  my  "Plasmoly- 
tische  Studien  ilber  die  Wand  der  Vacuolen."^  They 
teach  that,  in  artificial  constrictions  of  a  protoplast,  the 
limiting  membrane,  the  wall  of  the  vacuole,  and  the  gran- 
ular plasm  close  their  edges,  apparently  without  any  dif- 
ficulty, and  round  off  to  form  a  new  unit.  In  plasmolytic 
experiments  this  is  easily  verified.  Here  one  sees  also, 
how  upon  the  restoration  of  turgor,  the  parts  flow  to- 
gether again,  their  members  uniting  with  the  correspond- 
ing organs  of  the  other  parts  of  the  same  protoplast. 

This  power  of  combining  with  homologous  parts 
seems  to  be  universally  inherent  in  the  three  mentioned 
organs  of  the  plant-protoplast.  The  walls  of  the  vacu- 
oles  show  it  wherever  the  numerous  vesicles  of  cell-sap 
.  Wiss.  Bot.  16:  501-505.  1885. 


Regeneration  of  Protoplasts  After  Wounding     139 

in  young  tissue-cells  combine  into  one  large  vacuole  dur- 
ing the  rapid  growth  in  the  transition  to  the  adult  con- 
dition. When  two  or  more  like  protoplasts  unite  to  form 
a  so-called  symplast,  something  similar  takes  place  in 
their  walls,  at  least  in  some  cases,  as  in  the  plasmatic 
membrane  and  the  granular  plasm.  The  ontogeny  of  the 
latex-vessels  teaches  this  more  clearly  than  anything  else. 
A  fusion  of  like  parts  in  the  "feet"  of  many  rhizopods  has 
also  been  repeatedly  observed  and  described. 

As  far  as  we  know,  only  simple  contact  is  needed  for 
this  fusion,  besides  the  required  degree  of  homogenity. 
We  may,  therefore,  regard  it  as  a  mechanical  process  and 
use  it  as  an  element  in  the  explanation  of  normal  cell- 
division.  In  Spirogyra  it  evidently  accomplishes  the  fu- 
sion of  the  spindle  with  the  inward  growing  ring,  and 
later  determines  the  final  closing  up  of  the  opening  that 
was  left  in  the  ring. 

§  4.     The  Regeneration  of  Protoplasts  after  Wounding 

Even  though,  in  the  normal  course  of  development, 
the  individual  organs  of  a  cell  multiply  by  division,  this 
does  not  necessarily  imply  that  this  rule  must  be  without 
exception,  and  that  there  cannot  be  cases  where  nature 
tries  to  achieve  its  ends  in  another  way.  Especially  where, 
through  outward  interference,  such  as  wounding  and  mu- 
tilations, individual  members  of  a  protoplast  are  com- 
pletely lost,  it  might  be  expected  that  a  regeneration  in 
another  way  might  be  possible. 

To  be  sure  observations  now  available  do  not  warrant 
the  assumption  that  such  cases  actually  occur.  But  this 
does  not,  by  any  means,  exclude  their  possibility.  And 
on  this  possibility  I  want  to  lay  great  stress  in  this  con- 
nection, for  the  hypothesis  of  intracellular  pangenesis 


140          Historical  and  Critical  Considerations 

allows  us  to  regard  as  possible  an  occasional  neogenesis 
of  such  organs  out  of  pangens  proceeding  from  the  nu- 
cleus. 

Judging  from  the  facts  published  up  to  the  present 
time,  however,  the  phenomena  of  regeneration  after 
wounding  are  closely  connected  with  the  normal  pro- 
cesses. Nobody,  at  least  recently,  has  maintained  that  in 
such  a  case  there  is  a  new  formation  of  nucleus  and  chro- 
matophores.  There  have  been  only  few  investigations  in 
regard  to  a  possible  occurrence  of  new  vacuoles.  These 
were  made  by  Went  for  the  very  purpose  of  testing  the 
point  in  question,  and  teach  at  least  one  thing  with  cer- 
tainty, that  so  far,  wherever  it  had  been  thought  necessary 
to  assume  a  formation  de  novo  of  normal  vacuoles,  such 
does  not  really  take  place.  For  the  vacuoles  which  have 
been  observed  originate  partly  through  constriction  from 
the  large  sap- vesicle  of  the  cell,  and  partly  through  the 
swelling  of  the  smaller  ones  which  are  suspended  in  the 
granular  plasm.  Especially  in  the  case  of  the  Vaucheria, 
which  was  studied  first  by  Hanstein,  and  later  by  so  many 
investigators,  there  surely  can  no  longer  be  a  well  founded 
doubt  on  this  point.22 

Since  the  time  when,  in  my  "Plasmolytische  Studicn" 
I  expressed  the  opinion  and  sought  to  establish  the  fact 
that  the  plasmatic  membrane  is  a  separate  organ  of  the 
protoplast23  no  decisive  facts  on  this  subject  have  been 
published.  Klebs  is  opposed  to  my  assumption  on  the 
ground  of  an  observation  made  on  Vaucheria?*  For  the 
study  of  these  processes  this  investigator  introduced  a 
new  method,  which  makes  it  possible  to  demonstrate, 
easily  and  with  certainty,  the  beginnings  of  the  formation 

22Went,  F.  A.  F.  C.  Jahrb.  Wiss.  Bot.  19:    330-341.     1888. 

™jahrb.  Wiss.  Bot.  16:    493.     1885. 

**  Arbeit  en  Bot.  fust.,  Tubingen.    2:    510. 


Regeneration  of  Protoplasts  After  Wounding     141 

of  a  cell-membrane  around  exuded  masses  of  protoplasm. 
He  stains  the  water  or  the  diluted  solution  in  which  the 
threads  are  cut  through,  with  Congo-red,  which  is  stored 
up  with  great  avidity  by  these  young  cell-membranes. 

Nevertheless  this  method  does  not  yet  decide  the  ques- 
tion raised  by  me,  because,  as  Klebs  also  says,  there  is  no 
means  of  deciding  the  presence  or  absence  of  a  plasmatic 
membrane  on  a  portion  of  the  mutilated  protoplast  that 
forms  a  cell-membrane.  "Among  the  free  swimming  balls 
of  protoplasm  there  are  always  a  number  of  such  that  are 
quite  large  and  rich  in  contents  which  live  several  days 
but  without  forming  a  cell-membrane."  In  the  case  of 
most  of  them,  however,  the  beginnings  of  the  formation 
of  a  cell-membrane  are  very  soon  evident.25  Wherein 
the  difference  in  the  behavior  of  these  two  kinds  of  frac- 
tional parts  consists,  was  not  further  investigated  by 
Klebs.  My  assumption  that  the  former  lacked  the  limiting 
membrane,  while  the  latter  got  a  part  of  this  organ  when 
cut  off,  has  not  been  at  all  disproved. 

Nor  does  the  great  extensibility  of  the  plasmatic  mem- 
branes during  the  enormous  swelling  of  the  vesicles  which 
later  form  the  cell-membrane  seem  to  me  by  any  means 
improbable  or  even  surprising.  Plasmolytic  experiments 
teach  us  at  every  step  that  the  extensibility,  not  only  of 
the  plasmatic  membrane,  but  also  of  the  wall  of  the  vacu-, 
oles  and  perhaps  even  of  the  granular  plasm  is  very  con- 
siderable. And  Went  has  comprehensively  demonstrated 
that  the  swollen  spheres  of  Vaucheria  contain  only  such 
vacuoles  as  have  originated  by  the  enlargement,  and 
mostly  also  by  division  of  the  sap-vesicles  present  in  the 
uninjured  plant.  The  assumption  of  an  extensibility  of 
the  plasmatic  membrane  which  need  not  be  much  greater 
than  the  proven  elasticity  of  the  wall  of  the  vacuoles  can- 

*r*Loc.  dt.  p.  507. 


142          Historical  and  Critical  Considerations 

not  seem  very  surprising.  The  phenomena  of  regenera- 
tion of  Vaucheria  demand  renewed  investigation  in  this 
respect  also.  As  long  therefore,  as  there  is  no  actual 
proof  of  a  neogenesis  of  this  organ,  independently  of  the 
old  one,  we  cannot  recognize  such  great  significance  in 
this  instance  as  some  authorities  attribute  to  it. 

Here  also  the  observations  by  Haberlandt26  on  the 
same  phenomenon  are  important.  This  investigator  di- 
rected his  attention  chiefly  to  the  nuclei,  and  familiarized 
himself  with  their  behavior  during  regeneration.  The 
nuclei  accumulate  near  the  wound  in  the  plasma  deprived 
of  chlorophyll  bodies,  and  are  evidently  more  important 
than  the  latter  for  the  growth  of  the  new  cell-membrane. 
In  the  exuded  globules  of  protoplasm  which  remained 
alive,  Haberlandt  succeeded  almost  always  in  demon- 
strating the  presence  of  one  or  more  nuclei,  but  never  the 
absence  of  any.  In  spite  of  this,  not  all  of  them  formed 
a  new  cell-wall.  "At  times  there  occur  cell-forms  devoid 
of  a  membrane  and  rich  in  plasm.  If  the  sap-cavity  is 
lacking,  the  chlorophyll-bodies  aggregate  in  the  center, 
and  the  nuclei  lie  in  the  peripheral,  colorless  plasma.  In 
case  a  cavity  for  cell-sap  is  present,  the  chlorophyll-grains 
lie  in  the  innermost  layer  of  the  plasma-body  the  nuclei 
more  toward  the  outside."27  The  possession  of  nuclei  is 
therefore,  in  itself,  not  sufficient  for  the  formation  of  a 
cell-membrane.  It  would  be  important  to  find  out  whether 
the  parts  of  plasma  referred  to  are  perhaps  the  very  ones 
that  did  not  get  part  of  the  old  limiting  membrane. 

It  seems  to  me  to  be  of  great  interest  to  regard  the 
whole  pending  question  from  another  point  of  view,  and 
one  which  has  already  been  considered  by  Haberlandt. 

26Haberlandt,  G.    Ueber  die  Beziehungen  zwischen  Funktion  und 
Lage  des  Zellkernes.    pp.  83-97.    Jena,  1887. 
27Loc.  cit.  p.  92. 


Regeneration  of  Protoplasts  After  Wounding     143 

Regeneration  is  obviously  an  adaptation  to  guard  against 
the  results  of  injuries  which  occur  frequently  in  nature. 
In  such  cases  the  higher  plants  usually  give  up  the  affected 
cells;  the  large-celled  algae  and  fungi,  especially  those 
that  have  been  designated  by  Sachs  as  non-cellular,  evi- 
dently cannot  do  that.  Therefore  one  generally  finds  in 
them  the  power  of  closing  up  wounds.  That  it  would, 
however,  be  of  particular  importance  to  keep  escaped 
globules  of  protoplasm  alive  is  the  less  probable,  as  it  is 
only  possible  to  do  so  in  solutions  which  are  quite  a  little 
more  concentrated  than  those  in  which  the  respective 
plants  naturally  live.  Therefore,  the  closing  up  of  the 
wound  is  primary,  the  processes  in  the  escaped  plasma 
secondary.  From  the  adaptive  characters  available  for 
the  first,  it  ought  to  be  possible  to  explain  the  latter.  And 
as  long  as  the  first  can  be  explained  without  the  hypothe- 
sis of  an  independent  neogenesis  of  the  plasmatic  mem- 
brane, this  assumption  must  be  regarded  as  at  least  im- 
probable for  the  latter. 

This  consideration  leads  us  to  include  in  the  field  of 
these  studies  even  the  closing  up  of  wounds  in  latex-tubes. 
The  investigations  of  Schmidt  on  the  latex-vessels,  and  of 
Schwendener  on  the  latex-cells  may  serve  as  important 
points  of  departure  in  this.28  For  they  teach  that  in  parts 
of  latex-tubes  which  adjoin  the  wound  of  the  cut,  a  closing 
up  of  the  tube  can  be  accomplished  in  the  same  way  as  in 
some  Siphoneae  (e.  g.,  Bryopsis,  Codium,  Derbesia)  and 
in  many  pollen-tubes  the  injured  part  of  the  cell-cavity 
is  separated  from  the  uninjured  one.29 

28 Schmidt,  E.  Ueber  den  Plasmakorper  der  Geliederten  Milchroh- 
ren.  Bot.  Zeit.  40:  462.  1882.  -Schwendener,  S.  Einige  Beobach- 
tungen  an  Milchsaftgefassen.  Sitsungsb.  Kais.  Akad.  Wiss.  Berlin. 
20:  323.  1885. 

29Schmidt,  E.  he.  cit.  p.  462. 


CHAPTER  III 

THE  AUTONOMY  OF  THE  INDIVIDUAL  ORGANS  OF  THE 
PROTOPLASTS 

§  5.    Nucleus  and  Trophoplast 

A  review  of  our  knowledge  concerning  the  anatomy 
of  the  nucleus  can  be  regarded  as  superfluous  in  this  con- 
nection. This  knowledge  is  to  be  looked  upon  at  present 
as  an  established  achievement  of  science,  the  significance 
of  which  for  the  theory  of  heredity  can  hardly  be  doubted 
any  longer.  Flemming  in  the  zoological,  Strasburger  and 
Schmitz  in  the  botanical  field  have  broken  the  way,  and 
their  observations  have  been  verified  and  extended  in  the 
main  by  numerous  other  investigators. 

It  does  not  seem  to  be  quite  fully  decided  whether  the 
amitotic  nuclei,  which  have  originated  through  constriction 
and  scission,  are  of  significance  in  questions  of  heredity, 
or  whether  they  occur  in  somatic  cells  only,  and  not  on 
the  germ-tracks.  In  Chara  the  nuclei  in  the  apical  cells 
divide,  according  to  Johow's  investigations,  according  to 
the  usual  scheme  of  indirect  nuclear  division ;  the  smaller 
cells  of  the  grown  plant,  for  example  in  the  nodes,  remain 
forever  uni-nucleate,  while  the  larger  ones  become  multi- 
nucleate  through  constriction.  This  kind  of  nuclear  form- 
ation, however,  is  never  followed  by  cell-division.30  Ac- 
cording to  Zimmermann  direct  nuclear  division  in  the 
plant-world  "is  limited  to  only  those  cases  in  which  the 
nuclear  division  is  not  accompanied  by  cell-division."31 
'  30Johow,  F.  Die  Zellkerne  von  Chara  foetida,  Bot.  Zeit.  31: 
729.  1881. 

31Zimmermann  A.  Morphologic  und  Physiologic  der  Pflansen- 
zelle.  p.  34. 


Nucleus  and  Trophoplast  145 

In  the  multi-nucleat  cells  of  Valonla  Schmitz82  has  fre- 
quently observed  division,  and  always  observed  it  to  take 
place  by  constriction.  It  does  not  seem  to  be  established 
with  certainty,  for  all  cases,  how  the  nuclei  of  the  swarm- 
spores  originate  here  and  in  the  case  of  the  other  Siphono- 
cladiaceae,  whether  through  direct  or  indirect  division. 

In  this  connection  it  should  be  mentioned  that,  accord- 
ing to  Van  Beneden  and  Julin,  direct  and  karyokinetic 
nuclear  divisions  alternate  in  the  spermatogenesis  of  As- 
carls  inegalocephala.^  Thus  we  see  that  this  subject  is 
not  yet  ripe  for  theoretical  use. 

The  amyloplasts,  with  all  their  derivatives,  among 
which  the  chlorophyll  bodies  are  the  most  important,  Ar- 
thur Meyer  calls  trophoplasts.  In  the  lowest  plants  they 
are  not  yet  differentiated,  and,  as  far  as  these  belong  to 
the  Phycochromacese,  the  whole  non-nucleated  protoplasm 
of  the  cell,  according  to  Schmitz,  is  stained.34  But  later 
Hansgirg  demonstrated  nuclei  and  chromatophores  in 
some  algae  of  this  group.85  From  the  Chlorophycese  up- 
ward they  are  universal  in  the  green  plants.  In  the  higher 
plants,  where  they  were  discovered  by  Schimper,86  they 
are  usually  colorless  in  young  cells.  As  a  rule  they  re- 
main so  in  the  underground  parts,  which  are  normally  not 
exposed  to  light. 

Phylogenetically,  therefore,  plants  with  undifferen- 
tiated  colored  protoplasm  are  probably  older  than  those 

32 Schmitz,  F.  Die  vielkernigen  Zellen  der  Siphonocladiaceen.  p. 
27.  1879. 

83Van  Beneden  et  Julin,  La  spermatogenese  chez  I'Ascaride  me- 
galocephale,  Bruxelles,  1884. 

84 Schmitz,  F.  Die  Chromatophoren  der  Algen.  p>  9.    1882. 

8BHansgirg,  A.  Ber.  Deut.  Bot.  Ges.  3:    14.    1885. 

86Schimper,  A.  F.  W.  Ueber  die  Entwickelung  der  Chlorophyl- 
korner  und  Farbkorper.  Bot.  Zeit.  41:  105,  121,  137,  153.  1883. 


146  Autonomy  of  Cell-Organs 

which  possess  special  chromatophores.  Hence  we  must 
imagine  them  to  have  originated  from  the  others  through 
differentiation.  A  further  step  in  the  differentiation  is 
then  the  development  of  colorless  conditions  of  these 
chromatophores.  These  are  still  lacking  in  the  lower  Al- 
gae, occur  first  in  the  highest  groups  of  this  class,  and  at- 
tain their  full  significance  only  in  the  higher  plants.  In 
other  words,  we  must  regard  the  amyloplasts,  although 
they  are  generally  the  young  condition  from  which  chlor- 
ophyll bodies  develop,  as  the  consequences  of  a  higher 
differentiation  and  assume  that  they  have  developed  phylo- 
genetically  from  the  latter.  This  discussion  is  important 
for  the  reason  that  it  brings  nearer  to  our  understanding 
the  not  infrequent  changes  of  form  of  the  trophoplasts  on 
the  germ-tracks.  On  the  whole,  the  cells  of  the  germ- 
tracks  of  the  higher  plants  are,  as  many  authors  empha- 
size, of  an  embryonic  nature,  and  such  cells  probably 
always  possess  colorless  trophoplasts.  But  according  to 
our  definition  of  the  germ-tracks,  there  are  many  excep- 
tions to  this  rule.  Thus,  to  name  only  one  instance,  the 
prothallia  of  ferns,  in  their  youthful  state,  consist  of 
green,  dividing  cells,  with  well-formed  chlorophyll-grains, 
from  which  later  the  amyloplasts  of  the  egg-cells  will 
originate.  Also  in  the  callus-formation  of  cut  petioles  of 
Begonia,  Peperomia,  and  other  species,  a  reversion  of 
green  trophoplasts  into  colorless  ones  may  take  place, 
especially  in  the  case  of  the  production  of  adventitious 
buds.  And,  since  generally  the  amyloplasts  occur  in  young 
cells  and  their  derivates  in  grown  protoplasts,  these  and 
similar  cases  would  be  illustrative  of  a  pronounced  reju- 
venation. 

'  On  the  germ-tracks  the  amyloplasts  usually  take  on  a 
simple  roundish  form,  on  the  somatic  tracks  they  change 


Autonomy  of  Chromoplasts  147 

their  shape  considerably,  and  with  it  the  structure  and  size 
of  the  starch-grains  produced  by  them. 

Among  the  most  peculiar  characters  of  the  chromato- 
phores  in  connection  with  the  organization  of  the  proto- 
plasts, belong  their  autonomous  movements.  Since  the 
researches  of  Sachs  on  this  subject,  we  know  that  the 
chlorophyll  grains  of  some  plants  are  moved  about  by 
streams  of  the  granular  plasm  in  such  a  way  that,  under 
the  influence  of  light,  they  take  up  positions  which  are 
favorable  for  the  assimilation  of  carbon  dioxide.  But  in 
this  process  they  are  passive.  The  beautiful  researches  of 
Stahl,  however,  have  disclosed  independent  movements 
of  these  structures  under  the  influence  of  the  same  stimu- 
lation. They  consist  chiefly  in  changes  of  shape,  through 
which  the  organs  in  question  either  approach  a  more  or 
less  globular  shape,  or  that  of  a  flat,  circular  disc.  Thus 
it  is  brought  about  that,  in  direct  sunlight,  they  present 
a  smaller,  in  diffuse  daylight,  a  larger  surface  for  re- 
ceiving the  rays.  And  to  us  they  afford  an  insight  into 
the  high  degree  of  their  inner  differentiation  such  as  we 
could  never  have  attained  by  the  simpler  study  of  their 
chemical  activity. 

According  to  Weiss,  the  yellow  and  orange  chromo- 
plasts  at  times  also  make  autonomous  movements,  which, 
according  to  the  descriptions  of  this  author,  resemble  the 
changes  of  form  of  the  amoeba  and  the  white  blood-cor- 
puscles.37 These  structures,  therefore,  may  also  be  more 
highly  organized,  and  play  a  more  important  role,  than 
that  of  the  simple  task  of  giving  their  color  to  the  respec- 
tive plants. 

I  wish  to  lay  quite  particular  stress  here  on  these 

87Weiss,  A.  Ueber  spontane  Bewegungen  und  Formanderungen 
von  Farbstoffkorpern.  Sitzungsb.  Kais.  Akad.  Wiss.  Wien.  90:  1884. 


148  Autonomy  of  Cell-Organs 

phenomena,  for  up  to  the  present  time  they  have  probably 
not  been  utilized  for  the  theory  of  heredity.  But  the  more 
plainly  we  see  the  independence  of  the  individual  organs 
of  the  protoplasts,  and  the  more  clearly  our  conviction 
grows  that  they  require  a  high  inner  differentiation  for 
exercising  their  functions,  the  more  will  we  be  inclined 
to  give  them,  their  due  place  in  our  theory,  and  especially 
will  we  try  to  investigate  the  more  thoroughly  their  rela- 
tion to  the  hereditary  factors  accumulated  in  the  nucleus. 

Wherever,  hitherto,  we  have  succeeded  in  demonstrat- 
ing with  complete  certainty  the  origination  of  trophoplasts, 
we  have  found  that  they  arise  through  a  division  of  those 
already  present.  That  the  chlorophyll  grains,  in  the 
higher  plants  as  well  as  in  the  algae,  can  multiply  through 
constriction  and  scission  has  long  been  known.  But  it 
was  Schmitz  who  showed  that  this  process  is  the  only  form 
of  their  multiplication  in  the  algae.38  In  the  Characeae  he 
discovered,  in  the  apical  cells,  the  colorless  bodies  from 
which  the  green  organs  of  these  plants  are  derived  in 
the  same  way.  These  investigations  are  now  so  generally 
known  that  it  would  be  superfluous  to  describe  them  here 
in  detail.  I  shall  only  emphasize,  as  especially  important, 
the  fact  that  the  swarm-spores  also  possess  only  such 
chromatophores  as  they  have  received  from  their  mother- 
cell,  a  fact  that  was  especially  mentioned  in  the  case  of 
Cladophora  and  Halosphaera.39 

The  investigations  by  Schimper  and  others,  who  dis- 
covered this  same  law  for  the  phanerogams,  have  already 
been  discussed  in  one  of  the  preceding  Chapters. 

Special  consideration  is  still  due  to  the  rarer  forms 
derived  from  the  more  general  chromatophores.  In  the 

38Schmitz,  Die  Chromatophoren  der  Algen.  1882. 
*»Loc.  cit.  pp.  135,  136. 


Formation  of  Oil  149 

first  place  we  must  mention  the  eye-spot40  observed  in 
many  swarm-pores,  and  which,  according  to  the  opinion 
of  those  investigators  who  have  examined  it  more  care- 
fully, is  probably  a  metamorphosed  chromatophore,  the 
same  as  the  chromatic  bodies  of  the  higher  plants  studied 
by  Arthur  Meyer.41  In  the  Euglenae  its  origin  has  been 
more  carefully  studied  by  Klebs.  Here  it  always  origi- 
nates by  division,  the  organs  being  always  preserved  in 
the  resting  cells.42  It  is  not  yet  definitely  decided  whether 
or  not  the  pyrenoids  in  the  chorophyll  bodies  of  Spiro- 
gyra  and  other  algae  are  to  be  regarded  as  specially  dif- 
ferentiated parts  of  these  organs.  But  it  seems  certain 
that,  at  least  in  isolated  cases,  they  multiply  through  di- 
vision.43 

On  the  origination  of  oil  in  plant-cells  little  i«  known 
with  certainty.  Pfeffer  has  demonstrated  that  the  oil 
does  not  form  in  the  vacuoles,  but  lies  imbedded  in  the 
granular  plasm.  Special  organs  which  accumulate  it 
within  themselves  have  lately  been  described  by  Wakker 
for  Vanilla  planifolia,  and  have  been  called  elaioplasts. 
Although  it  has  not  been  possible  to  find  out  their  mode 
of  origin,  the  most  natural  assumption  is  that  they  are 
metamorphosed  chromatophores.44  In  some  cases,  as  for 
example  in  the  diatomes,  the  oil-drops  of  the  Algae  evi- 

40Cf.  Zimmerman,  Die  Morphologic  und  Physiologic  der  Pflan- 
zenzelle.  p.  71.  1887. 

41Meyer,  Arthur,  Das  Clorophyllkorn.    1883. 

42Klebs,  Ueber  die  Organisation  einiger  Flagellatengruppen. 
Unters.  Bot.  Inst.  Tubingen.  1:  233. 

43Schmitz,  F.  Die  Chromotophoren  der  Algen.  pp.  42  and  65. 
1882.  Schmitz,  F.  Beitrage  zur  Kentniss  der  Chromatophoren. 
Jahrb.  Wiss.  Bot.  15:  142.  1884.  Strasburger,  E.  Ueber  Kern-  und 
Zelltheilung.  p.  26.  1888. 

'4tWakker,  J.  H.  De  Elaioplast.  Maandbl.  v.  Natuurwetensch. 
No.  8.  1887. 


150  Autonomy  of  Cell-Organs 

dently  do  not  lie  in  the  chromatophores,  and  this,  accord- 
ing to  Schmitz,  is  a  general  rule.45  But  in  the  higher 
plants  this  seems  at  times  to  be  the  case.46 

Last  to  be  mentioned  here  are  the  microsomes.  In 
most  cases  it  seems  to  be  unknown  what  they  are.  Small 
oil-droplets,  starch-grains,  inactive  vacuoles,  amyloplasts, 
protein  bodies  formed  by  fixation47  through  the  coagula- 
tion of  the  protein  dissolved  in  the  protoplasm,  and  per- 
haps many  other  formations  are  frequently  all  classed 
under  this  name.  Very  justly  has  Strasburger  claimed 
"that  not  the  microsome  but  the  hyaloplasm  is  to  be  con- 
sidered the  active  substance."48  At  any  rate  it  ought 
never  to  be  forgotten  that  the  word  microsome  stands 
only  for  a  question  mark,  and  that  we  can  talk  of  an  in- 
sight into  the  significance  of  these  structures  only  after 
the  question  concerning  their  nature  in  the  cases  con- 
cerned shall  have  been  answered. 

§  6.  The  Vacuoles 

Vacuoles  were  formerly  regarded  as  empty  spaces  in 
the  interior  of  the  protoplasm.  This  accounts  for  their 
name,  and  explains  the  small  interest  shown  in  them,  until 
recently,  in  the  study  of  the  anatomy  of  the  cell.  It  is 
only  since  Sachs  discovered  that  the  turgidity  of  growing 
cells  is  not  due  to  an  imbibition  of  water  in  their  walls, 
as  was  previously  assumed,  but  to  an  osmotic  tension  be- 
tween the  wall  and  the  cell  sap,  that  attention  was  directed 
to  the  significance  of  the  vacuoles.49 

45 Schmitz.    Loc.  cit.  p.  164. 

46Cf.  Meyer,  Arthur.  Das  Chlorophyllkorn,  pp.  14  and  31.     1883. 

47  i.  e.  artifacts  caused  by  the  "fixing"  fluid.     Tr. 

48  Strasburger,  E.     Neue  Untersuchungen.  p.  107.    1884. 
49Sachs,  J.  von.  Lehrbuch  der  Botanik,  3  Aufl.     1872;  4.  Aufl. 

1874,  p.  757. 


Autonomy  of  Vacuoles  151 

% 

This  was  still  more  the  case  through  the  demonstra- 
tion furnished  by  the  same  author,  that  the  tension  to 
which  growing  cell-membranes  are  subjected  by  the  cell- 
sap  is  one  of  the  most  essential  mechanical  causes  of  the 
surface  growth  of  these  membranes.  For  with  this  dem- 
onstration Sachs  laid  the  foundation  still  valid,  for  the 
whole  mechanical  theory  of  growth  in  length. 

Building  on  this  foundation,  many  investigators  have 
enlarged  our  knowledge  of  the  mechanical  causes  of 
growth  in  various  directions.  Some  have  especially 
measured  and  analyzed  the  degree  of  extensibility  of  the 
cell-membranes  and  the  amount  of  force  supplied  by  the 
cell-sap.  Others  have  studied  the  causes  governing  the 
variations  of  extensibility  of  the  wall  in  one  and  the  same 
cell,  and  which  occur  in  different  spots  and  in  different 
directions,  and  have  explained  them,  as  due,  with  great 
probability,  to  local  differentiations  in  the  protoplast  it- 
self, which  might  regulate  this  elasticity  through  the 
secretion  of  certain  enzymes.  Others  again  have  at- 
tacked the  doctrine  of  intussusception,  which  was  the 
prevailing  one  at  the  time  of  the  discoveries  mentioned, 
have  proven  it  to  be  incorrect,  and  have  tried  to  ressusci- 
tate  in  its  place,  in  a  new  form,  the  old  "apposition 
theory." 

Although  subject  to  misunderstandings  from  some 
sides,50  Sach's  theory  has  acquired  a  prominent  position 
in  plant-physiology,  and,  since  the  two  decades  of  its  es- 
tablishment, it  has  become,  in  ever  increasing  measure, 

50In  my  "Untersuchungen  iiber  die  Mechanischen  Ursachen  der 
Zellstreckung"  (p.  3,  1877.),  I  have  distinctly  emphasized  the  fact 
that  there  are  also  phenomena  of  growth  independent  of  turgor, 
and  that  therefore  this  turgor  is  neither  the  only,  nor  even  the  first 
reason  for  growth.  Krabbe  and  Klebs  arrived  later  at  the  same 
conclusion.  Cf.  Arbeiten  Bot.  Inst.  Tubingen.  2:  530.  1888. 


152  Autonomy  of  Cell-Organs 

the  starting  point  of  new  investigations.  It  has  been, 
without  doubt,  one  of  the  most  fruitful  thoughts  for  the 
development  of  our  science. 

The  further  study  of  the  cell-sap  and  the  vacuoles, 
suggested  by  this  theory,  has  led  in  regard  to  the  morpho- 
logical aspect,  which  alone  interests  us  here,  to  the  proof 
that  the  wall  of  the  vacuoles  is  an  essential,  never  wanting 
part  of  the  plant-protoplast.51  The  method  which  made 
it  possible  always  to  demonstrate  the  presence  of  this  wall 
consisted  in  the  treatment  of  the  living  cells  with  a  10% 
solution  of  potassium  nitrate,  which  has  been  stained  with 
eosin.  Directly,  or  after  a  shorter  or  longer  period,  the 
outer  protoplasm  dies  in  the  reagent,  while  the  wall  of  the 
vacuoles  remains  living  for  a  while.  It  is  then  visible 
as  a  distended  bubble,  more  or  less  completely  separated 
from  the  dead  parts,  and  entirely  preventing  the  penetra- 
tion of  the  eosin.  In  colorless  cells,  therefore,  the  bubble 
carries  contents  as  clear  as  water,  while  the  remaining 
protoplasm  is  stained  red  or  brown  by  the  eosin.  Fre- 
quently the  original  vacuole  separates  into  several  smaller 
ones;  and  not  infrequently  one  can  follow  this  process 
directly  under  the  microscope. 

The  wall  of  the  vacuoles  is  to  be  regarded  as  a  special 
organ  of  the  protoplast,  which  regulates  the  secretion  and 
accumulation  of  the  substances  which  are  present  in  the 
cell-sap  in  solution,  and  because  of  this  function,  it  has 
been  given  the  name  tonoplast.  But  frequently  the  sap- 
spaces  together  with  their  walls  are  now  designated  as  vac- 
uoles. 

In  living  cells  the  tonoplasts  are,  as  a  rule,  not  visible, 
because  they  consist  of  translucent  vesicles  of  an  extreme 

61Vries,  H.  de.  Plasmolytische  Studien  iiber  die  Wand  der  Vac- 
uolen.  Jahrb.  Wiss.  Bot.  16:  465.  1885. 


Autonomy  of  Vacuoles  153 

thinness.  But  they  are  clearly  and  distinctly  visible  in  the 
tentacle-cells  of  some  insectivorous  plants,  especially  of 
the  Drosera  rotundifolia  and  D.  intermedia.  The  process 
of  aggregation,  discovered  by  Darwin,52  taking  place  here 
during  the  digestion  of  the  prey,  belongs  to  the  most  in- 
teresting phenomena  that  the  life  of  a  cell  presents  for 
our  admiration.  In  the  resting  tentacle-cells  there  lies 
usually  a  large  vacuole  containing  red  cell-sap.  Under 
the  influence  of  stimulation  it  separates  into  several,  and 
soon  into  numerous  smaller  ones.  These  contract,  while 
secreting  part  of  their  contents,  and  are  now  carried 
through  the  cells  by  the  currents  of  the  granular  plasm, 
with  great  rapidity,  and  in  the  most  various  directions. 
Thus  they  lie  as  red  vesicles  in  unstained  sub- 
stance, and  can  therefore  be  seen  very  distinctly.  Dur- 
ing these  movements  they  undergo  striking  changes  of 
form ;  sometimes  they  are  drawn  out  into  long  tubes,  and 
thereupon  split  into  numerous  small  globules,  sometimes 
two  or  more  unite  to  form  larger  vesicles.  Toward  the 
end  of  the  phenomenon  this  last  process  has  the  pre- 
cedence, and  finally  all  the  sap-bubbles  have  again  united 
into  one,  of  the  original  volume.53 

The  above  mentioned  phenomena  of  aggregation,  and 
the  division  of  the  vacuoles,  as  it  is  so  frequently  ob- 
served in  plasmolysis  placed  the  ability  of  these  organs 
to  multiply  by  this  process  beyond  any  doubt.  From  the 
analogy  of  these  structures  with  the  chromatophores  I 
then  deduced  the  assumption,  that  "like  the  amyloplasts, 
they  can  be  produced  in  no  other  way  than  by  division."54 

52Darwin,  C.    Insectivorous  Plants.    Chap.  III.        1875. 

53Vries,  H.  de.  Ueber  die  Aggregation  im  Protoplasma  von 
Drosera  rotundifolia.  Bot.  Zeit.  44:  1,  17,  33,  57.  1886. 

54Vries,  H.  de.  Plasmolytische  Studien  iiber  die  Wand  der  Vac- 
uolen.  Jahrb.  Wiss.  Bot.  16:  505.  1885. 


154  Autonomy  of  Cell-Organs 

This  supposition  has  since  been  completely  confirmed 
by  Went.55  He  showed  first,  that,  contrary  to  the  pre- 
vailing opinion,  vacuoles  are  present  even  in  the  youngest 
cells  of  the  meristerrk  These  multiply  continuously 
through  division,  and  observation  teaches  that  during 
cell-division  one-half  of  the  vacuoles  present  goes  to  one 
daughter-cell  and  the  other  half  to  the  other.  Some- 
times it  was  possible  to  observe  the  constriction  and  after- 
wards the  transmission  of  the  two  sap-vesicles,  formed 
in  this  way,  to  the  daughter-cells.  From  the  vacuoles 
of  the  meristem  all  the  vacuoles  of  the  entire  plant  can 
thus  be  derived.  Divisions  of  these  structures  are  to  be 
found  everywhere ;  formations  de  novo  nowhere.  In  the 
same  way,  in  the  cryptogams  that  grow  with  an  apical 
cell,  all  the  vacuoles  originate  from  the  original  vesicles 
present  in  these  cells. 

According  to  these  investigations  the  vacuoles  behave 
exactly  in  the  same  way  as  the  chromatophores,  and  are 
just  as  independent  cell-structures  as  the  latter.  And 
through  the  demonstration  of  this  independence,  the  pan- 
meristic  conception  of  cell-division  has  been  definitely 
proven  as  correct,  in  opposition  to  the  former  neogenetic 
one. 

According  to  later  communication  by  the  same  author, 
he  succeeded  also  in  observing  the  formation  of  vacuoles 
in  some  special  cases  which  had  not  been  studied  before. 
Here  should  be  emphasized  the  formation  of  these  organs 
in  the  swarm-spores  which,  according  to  a  communication 
by  letter  from  Went,  comes  about  by  a  division  of  the 
sap-vesicle  in  the  mother-cell  in  such  a  way  that  every 


55Went,  F.  A.  F.  C.     Die  Vermehrung  der  normalen  Vacuolen 
durch  Theilung.    Jahrb.  Wiss.  Bot.  19:  295.    1888. 


Autonomy  of  Vacuoles  155 

swarmer  receives  into  its  body  a  portion  divided  off  from 
this  bubble. 

In  the  literature,  an  origination  of  sap-cavities  in  nu- 
clei, chromatophores,  or  even  in  the  granular  plasm,  out- 
side the  vacuoles  already  present,  has  repeatedly  been 
described.  But,  on  investigating  these  cases,  it  was  found 
that  here  one  had  to  deal,  not  with  normal  vacuoles,  but 
with  pathological  formations,  which  occur  with  the  age- 
ing or  dying  of  the  cell.  Frequently  they  are  also  due 
to  the  influence  of  the  water  in  which  the  preparations  are 
examined.56 

From  the  theory  that  the  vacuoles  originate  only 
through  division,  it  may  be  concluded  that  the  sap-vesicles 
of  germinating  seeds  are  derived  from  those  present  in  the 
ripening  ovules,  and  that,  therefore,  in  the  ripe  condi- 
tion, the  vacuoles  must  indeed  be  dried  out,  but  cannot 
be  entirely  lacking.  Following  up  this  thought  Wakker  ar- 
rived at  the  noteworthy  discovery  that  the  aleuron-grains 
are  the  dry  states  of  the  vacuoles  in  the  seed.57  During  the 
process  of  ripening,  the  amount  of  protein  matter  dissolved 
in  the  cell-sap  gradually  increases  until  the  fluid  becomes  of 
a  thick,  slimy  consistency.  In  drying,  some  of  the  protein 
bodies  crystallize  and  form  the  well  known  crystalloids, 
while  the  remaining  protein  hardens  into  an  amorphous 
mass  around  them.  When  soaking  the  seed,  these  masses 
soften  gradually  and  are  later  utilized  as  nourishment. 
By  using  a  solution  of  one  part  nitric  acid  in  four  parts 

56Went,  F.  A.  F.  C.  De  jongste  toestanden  der  vacuolen,  pp. 
45-65. 

57Wakkcr,  J.  H.  Aleuronkorrels  zyn  vacuolen.  Maandbl.  r. 
Naturw.,  Nr.  5.  1887.  Over  kristalloiden  en  andere  lichamen  die 
in  de  cellen  van  zeuvieren  voorkomes.  Bot.  Cent.  33:  138.  1888, 
and  Jalwb.  Wiss.  Bot.  19:  423.  1888.  Since  that  time  this  result 
has  been  confirmed  by  Werminski,  Ber.  Deut.  Bot.  Ges.  6:  199.  1888. 


156  Autonomy  of  Cell-Organs 

of  water,  one  can  bring  about  at  will  this  hardening  in  the 
still  liquid  cell-sap,  and  in  this  way  artificially  produce  the 
formation  of  aleuron-grains  under  his  very  eyes. 

It  is  important  that,  in  some  seeds  more,  in  others 
less,  the  vacuoles  divide  during  the  process  of  ripening 
into  several  smaller,  frequently  into  very  numerous  ex- 
tremely minute  vesicles,  which  gradually  fuse  again  into 
one  large  vacuole  at  the  beginning  of  germination. 

The  processes  in  the  seed,  therefore,  fit  beautifully 
into  the  conception  that  the  vacuoles  originate  only  by 
division.58 

Just  as  the  chromatophores  can  differentiate  into  the 
most  various  organs,  so  also  can  the  vacuoles,  although 
to  a  lesser  extent.  Went  observed  how,  in  different  cells, 
there  lie  vacuoles  which  remain  separated  throughout 
their  existence,  and  are  distinguished  by  their  different 
contents.59  Frequently  some  of  them  are  stained,  others 
are  colorless,  or  some  contain  tannin,  which  is  lacking  in 
others.  In  such  cases  the  latter  are  called  by  that  author 
adventitious  vacuoles. 

The  contractile  or  pulsating  vacuoles  form  a  special 
system.  In  the  swarm-spores  of  the  algae  they  probably 
originate  from  the  other  vacuoles60  through  further  dif- 
ferentiation, but  in  the  Euglense,  according  to  the  investi- 

58In  Miiller's  bodies  of  the  ant-plant,  Cecropia  adenopus,  Schim- 
per  illustrates  formations  in  the  cell-contents  which,  at  first  glance, 
look  like  vacuoles,  and  which,  on  account  of  their  semi-fluid  con- 
tents, he  compares  with  the  aleuron-grains.  Their  origination  from 
vacuoles  can  hardly  be  doubted.  Schimper,  A.  F.  W.  Die  Wechsel- 
beziehungen  zivischen  Pfianzen  und  Ameisen.  1888.  Cf.  especially 
Taf.  II,  Fig.  11.  Also  Wakker,  Jahrb.  Wiss.  Bot.  19:  467.  1888. 

59Went.  loc.  cit.  pp.  65-91. 

60Or  have  the  turgor-vacuoles  possibly  originated  phylogenetically 
from  the  pulsating  ones? 


Autonomy  of  Plasmatic  Membranes  157 

gations  of  Klebs,  they  multiply  by  division.61  They 
possess  here  a  wall  of  their  own  which  resembles  the 
walls  of  ordinary  vacuoles  in  its  great  power  of  resistance. 
Klebs  observed  how  the  pulsation  may  continue  for  a 
long  time  after  the  rest  of  the  protoplast  has  been  killed 
by  some  mechanical  interference.  The  view  that,  in 
systole,  the  contents  of  these  vacuoles  are  expelled  into 
the  surrounding  tissues,  while,  in  diastole,  fluid  is  taken 
from  the  protoplast,  is  probably  generally  accepted  for 
rhizopods  and  flagellates.  My  own  observation  con- 
vinced me  of  its  correctness  in  Actinophrys  Sol.  The 
same  opinion  may  also  apply  to  the  pulsating  vacuoles  in 
the  plant-world.62 

§  7.   The  Relation  Between  the  Plasmatic  Membranes  and 
the  Granular  Plasm 

While  the  investigations  of  the  last  two  decades  have 
thrown  a  clear  light  on  the  organs  of  the  protoplasts  just 
discussed,  the  relation  between  plasmatic  membrane  and 
granular  plasm  is  still  completely  in  the  dark.  In  our 
knowledge  of  the  mode  of  origin  of  the  nuclei,  tropho- 
plasts,  and  vacuoles,  the  theory  of  heredity,  as  I  have 
tried  to  explain  in  this  Section,  finds  its  indispensable 
basis.  On  the  mutual  relation  of  the  two  other  men- 
tioned parts  of  the  protoplast,  no  facts  have  yet  been 
found,  which  might  be  utilized  for  the  theory. 

As  already  mentioned,  what  the  nature  of  that  relation 
is,  is  certainly  not  of  essential  importance  for  the 
hypothesis  of  intracellular  pangenesis.  Yet  it  remains  an 
important  question  whether  granular  plasm  and  plasmatic 
membrane  are  mutually  as  independent  as  the  granular 

61Klebs,  G.    Arbeiten  Bot.  Inst.   Tubingen,  Bd.  I.  p.  250.  ff. 
62Pfeffer,  Pflanzenphysiologie,  pp.  399-401. 


158  Autonomy  of  Cell-Organs 

plasm  and  the  wall  of  the  vacuole,  or  whether  they  stand 
in  the  same  genetic  relation  as  amyloplasts  and  chloro- 
phyll-grains. As  long  as  this  question  remains  undecided, 
the  application  of  my  hypothesis  to  the  plasmatic  mem- 
brane and  therewith  to  the  surface  growth  of  the  cell- 
membrane  and  all  the  formative  processes  of  the  cells, 
is  rendered  very  difficult.  For  this  reason  may  I  be 
allowed  to  subject  the  respective  phenomena  to  a  critical 
revision  in  order  to  encourage  further  research.  I  think 
it  will  then  be  seen  that  the  prevailing  opinion  that  the 
plasmatic  membrane  originates  in  every  case  from  the 
granular  plasm  is,  for  the  present,  not  supported  by  cer- 
tain and  closely  observed  facts,  but  is  adhered  to  only 
from  habit.  This,  however,  it  seems  to  me,  ought  not 
to  be  allowed  in  view  of  the  newer  knowledge  in  regard 
to  the  origin  of  the  wall  of  the  vacuole.  For,  as  long  as 
no  special  wall  was  assumed  for  the  vacuoles,  it  was  nat- 
ural not  to  regard  the  plasmatic  membrane  as  a  special 
organ.  Since  the  independence  of  the  former  has  been 
established,  such  is  obviously  most  probably  the  case  for 
the  latter  also.63 

Besides  the  incompleteness  of  the  observations,  which 
is  to  be  demonstrated  in  the  next  paragraph,  the  whole 
course  of  the  development  of  our  knowledge  in  the  field  of 
cell-anatomy  on  the  one  hand,  and  the  already  repeatedly 
described  differentiations  of  the  plasmatic  membrane  and 
the  granular  plasm  on  the  other  hand,  controvert  the 
prevailing  opinion.  The  latter  does  not  form  at  all,  as 

63A  method  by  which  the  plasmatic  membrane  could  be  arti- 
ficially separated  everywhere  from  the  granular  plasm,  just  as  strong 
plasmolytic  reagents  separate  the  wall  of  the  vacuole,  is  particu- 
larly desirable.  Such  a  method  could  also  render  great  service  in 
judging  the  hypothesis  mentioned  on  page  160,  Note  2,  on  the  growth 
in  thickness  of  the  cell-membranes. 


Autonomy  of  Plasmatic  Membranes  159 

the  old  conception  would  have  it,  a  ground-substance  of 
protoplasm,  mixing  constantly  by  its  movements,  and 
therefore  not  organized  in  the  ordinary  sense.  This  is 
most  clearly  seen  in  the  Characeae.  Here  it  consists,  first 
of  all,  of  a  moving  portion  and  of  a  resting  part  that 
contains  the  chlorophyll  grains.  When,  sometimes  the 
green  plastids  are  torn  from  their  position,  and  carried 
away  by  the  current,  one  sees  that  they  did  not  adhere 
separately  to  the  plasmatic  membrane,  for  they  are  not 
carried  off  singly,  but  in  bands  and  groups,  while  within 
these  the  grains  retain  their  mutual  position  and  distance. 
Neither  does  the  moving  part  form  a  whole,  for  the  ra- 
pidity of  the  current  is  not  at  all  everywhere  the  same 
on  a  cross-section.  It  is  greater  near  the  chlorophyll- 
grains  than  next  to  the  wall  of  the  vacuole,  and  further- 
more it  increases  from  the  two  indifferent  zones  toward 
the  center  of  the  green  areas  which  are  separated  by  them. 
With  declining  vital  energy  the  more  torpid  currents  are 
the  first  to  suspend  movement,  while  the  more  rapid  ones 
continue  to  move,  and  with  decreasing  rapidity  the  width 
of  the  current  diminishes  at  the  same  time. 

Quite  generally  speaking,  the  granular  plasm  seems 
to  consist,  in  the  plant-world,  of  moving  and  of  resting 
parts,  the  limits  of  which  can  be  shifted  by  more  or  less 
favorable  life-conditions,  or  can  also  shift  spontaneously 
in  the  course  of  development,  adapting  themselves  to 
changing  needs. 

The  latter  condition  is  illustrated  by  the  beautiful  in- 
vestigations by  Dippel,  Criiger,  and  Strasburger  on  the 
relations  between  the  plasma-currents  and  the  internal 
sculpture  of  the  cell-wall.64  For  along  those  places  where 

e*Dippel.  Abhandl.  Naturf.  Gcs.,  Halle.  10:  55.  1864.  Criiger, 
H.  Westindische  Fragmente.  Bot.  Zeit.  13:  623.  1855.  Stras- 
burger, E.  Jenaische  Zeitschr.  Naturwiss.  10:  417.  1876. 


160  Autonomy  of  Cell-Organs 

ledges,  jutting  into  the  interior,  are  in  the  process  of  for- 
mation there  generally  run  strong  currents  which  evi- 
dently bring  and  distribute  the  requisite  food.  But  this 
differentiation  in  the  granular  plasm  is,  to  all  appearances, 
controlled  by  a  corresponding  differentiation  in  the  plas- 
matic  membrane.  For,  according  to  Dippel,  the  bands 
which  form  the  layers  of  cellulose,  consist  of  an  outer 
hyaline  band,  which  is  thicker  than  the  rest  of  the  plas- 
matic  membrane,  and,  like^  the  latter,  cannot  be  stained 
with  iodine,  together  with  an  inner,  moving  layer  of  the 
granule-bearing  plasm,  which  takes  a  deep  yellow  tint 
when  treated  with  iodine.65  The  hyaline  band  is  evi- 
dently a  differentiated  part  of  the  plasmatic  membrane 
which,  on  its  inside  is  covered  and  nourished  by  the  cur- 
rent, and  on  its  outside  forms  the  ledges  of  the  cell- 
membrane.66 

In  naked  protoplasts  the  cilia  also  bespeak  an  inner 
organization  of  the  plasmatic  membrane.  These  are  de- 
scribed by  Strasburger67  for  the  swarm-spores  of  Vau- 
cheria.  Here  all  the  cilia  adhere  to  a  denser  part  of  this 
layer;  they  appear  to  be  embedded  in  it  by  a  thick  root. 

§  8.    The  Question  of  the  Autonomy  of  the  Limiting 
Membrane 

While  in  cell-division,  according  to  the  type  described 
by  Mohl,  the  multiplication  of  the  limiting  membrane  by 

65Loc.  cit.  pp.  57,  58. 

66Strasburger's  hypothesis  that  the  growth  of  the  cell-wall  is 
accompanied  by  a  transformation  layer  by  layer  of  the  outermost 
strata  of  the  limiting  membrane  into  cell-wall  can,  without  difficulty 
be  combined  with  the  assumption  of  the  autonomy  of  this  organ  with 
reference  to  the  granular  plasm,  and  therefore  need  not  be  discussed 
in  detail  here. 

67 Strasburger,  Studien  uber  das  protoplasma,  p.  400.      1876. 


Autonomy  of  the  Limiting  Membrane  161 

division  and  growth  is  generally  recognized,  the  insertion 
of  a  new  layer  and  its  connection  with  the  old  membrane 
is  usually  assumed  for  cell-formation  in  the  higher  plants. 
In  addition  to  this,  there  are  some  cases  of  cell-formation 
which  seem  to  argue  quite  directly  in  favor  of  a  formation 
of  the  limiting  membrane  de  novo  from  the  granular 
plasm. 

All  these  cases  seem  urgently  to  demand  renewed  in- 
vestigation. It  is  only  with  the  intention  of  encouraging 
it  that  I  shall  briefly  discuss  them  here. 

In  regard  to  the  ordinary  mode  of  cell-division  the 
situation  has  greatly  changed  during  the  past  year  through 
a  discovery  by  Went68  which  has  been  confirmed  by  Stras- 
burger.69  This  discovery  concerns  the  nature  of  the  so- 
called  cell-plate,  which,  when  nuclear  division  is 
completed,  forms  at  the  equator  of  the  now  barrel-shaped 
figure.  As  the  name  indicates,  the  cell-plate  is  regarded 
as  a  layer  which,  cutting  across  the  figure,  later  divides 
into  two  layers,  and  between  these  secretes  the  new  cel- 
lulose lamella.  These  two  halves  of  the  layer  are  the  two 
complementary  pieces  of  the  plasmatic  membrane ;  as  the 
barrel  becomes  flattened  and  extends  laterally  toward  the 
cell-walls,  they  increase  until  they  reach  the  old  limiting 
membrane  of  the  mother-cell  and  blend  with  it. 

Went  succeeded  in  loosening  this  whole  division  fig- 
ure from  the  cells  after  they  had  been  fixed  and  stained, 
and  allowed  it  to  float  around  in  the  fluid  of  the  prepa- 
ration. In  this  way  it  became  possible,  by  turning  the 
cell-plate,  to  study  a  polar  view  of  it,  while  hitherto  only 
the  side-view  had  been  studied  and  figured.  As  long  as 

68Went,  F.  A.  F.  C.  Beobachtungen  iiber  Kern-und  Zell- 
theilung.  Ber.  Deut.  Bot.  Ges.  5:  247.  1887. 

69Strasburger,  Ueber  Kern-und  Zelltheilung.  1888. 


162  Autonomy  of  Cell-Organs 

the  cell-plate  is  smaller  than  the  daughter-nuclei,  this 
view,  of  course,  does  not  teach  anything,  because  it  has 
not  been  possible  to  remove  the  nuclei.  But  as  soon  as 
the  cell-plate  protrudes  sideways  from  betwen  the  nuclei, 
it  can  be  seen  that  it  is  not,  by  any  means,  a  continuous 
plate,  but  only  a  rather  thin  ring.  This  ring  lies  in  the 
connecting  tube  that  separates  the  interior  of  the  figure 
from  its  surroundings  and  has  probably  the  same  signifi- 
cance as  in  Spirogyra.™  This  "cell-ring,"  as  we  must  now 
call  the  cell-plate,  enlarges  until  it  unites,  first  on  one, 
then  gradually  on  all  sides,  with  the  peripheral  protoplasm 
of  the  mother-cell. 

That  the  plane  of  the  cell-ring  is  the  place  where  the 
dividing  wall  forms,  is  certain,  and  agrees  essentially  with 
the  previous  conception  of  the  cell-plate.  But  it  has  not 
yet  been  possible  to  discover  whether  or  not  the  secretion 
of  cellulose  in  the  cell-ring  begins  before  the  latter  has 
joined  the  wall  of  the  mother-cell  at  least  on  one  side.  As 
soon  as  its  presence  can  be  proven  by  reagents,  the  new 
membrane  is  already  joining  the  wall  of  the  mother-cell,  at 
least  on  one  side.71  Likewise  it  has  not  been  decided, 
whether,  in  the  plane  of  the  ring  there  is  extended  a  mem- 
brane which  crosses  the  vacuole  situated  there  and  sepa- 
rates it  into  two  separate  sap-vesicles.  But  this  is  not 
probable. 

It  is  clear  that,  with  the  discovery  of  the  cell-ring,  the 
old  conception  of  cell-division  that  contradicts  the  auton- 
omy of  the  plasmatic  membrane,  is  weakened.  For  its 
final  refutation,  however,  further  researches  are  neces- 
sary, especially  such  as  will  include  the  wall  of  the 
vacuoles  in  the  figures  of  division. 

70Cf.  pp.  132-134. 

72Strasburger,  E.  Bot.  Praktikum,  p.  597.  1884,  and  Ueber  Kern- 
und  Zelltheihmg.  p.  171  ff.  1888. 


Autonomy  of  the  Limiting  Membrane  163 

I  agree  here  with  Zacharias72  who,  from  observation  on 
Chara,  is  of  the  opinion  that  the  cell-plate  elements  origi- 
nate from  the  cytoplasm  surrounding  the  nuclear  figure. 
I  wish  also  to  recall  here  an  opinion  of  Flemming's,  ac- 
cording to  which,  cell-division  in  plants  and  animals 
generally  begins  with  a  constriction  of  the  protoplast. 
This  constriction  has  not  been  observed  in  many  prepa- 
rations for  the  only  reason  that  it  is  frequently  unilateral, 
and  therefore  requires  a  special  position  of  the  cell  under 
the  microscope  in  order  to  be  seen.78 

Platner's  view  that  the  spindle  fibers  are  currents  of 
the  granular  plasm  requires  further  investigation.  For 
this  purpose  direct  observation  on  the  living  object  is 
necessary.  Obviously  the  plasma-currents  have,  until 
now,  been  sadly  neglected  in  the  study  of  cell-division. 

There  are  still  left  for  us  to  consider  the  instances  of 
so-called  free  cell-formation,  which  probably  represent 
the  most  striking  exceptions  to  the  rule  of  the  autonomous 
origination  of  the  plasmatic  membrane.  By  free  cell- 
formation  is  meant  those  cases  in  which  not  all  of  the 
protoplast  of  the  mother-cell  is  used  in  the  formation  of 
the  daughter-cells.74  The  new  cells  were  thought  to  have 
originated  in  the  interior  of  the  mother-cell,  and  there- 
fore without  any  contact  with  the  limiting  membrane. 

72Zacharias,  E.  Ueber  Strasburger's  Schrift  Kern-und  Zell- 
theilung  im  Pflanzenreiche.  Jena.  1888.  Bot.  Zei't.  46:  456.  1888. 

73Flemming.  Zellsubstanz,  Kern-und  Zelltheilung.  p.  243. 
1882. 

74In  the  most  recent  interview  of  the  pertinent  literature,  Zim- 
mermann  suggests  that  the  name  free  cell-formation  be  not  used  for 
these  phenomena,  but  for  the  formation  of  free  cells,  i.  e.,  of  such  that 
lose  their  connection  with  the  mother-cell.  If  it  should  be  discovered 
that  a  free  cell-formation  in  the  old  sense,  does  not  exist  in  the  plant- 
world,  this  suggestion  would  certainly  be  acceptable.  Cf.  Die  Morph- 
ologie  und  Physiologie  dcr  Pfianzenzclle,  p.  160.  1887. 


164  Autonomy  of  Cell-Organs 

Hence  it  was  clear  that  their  limiting  membrane  must  have 
been  derived  from  the  granular  plasm.75 

In  the  formation  of  the  endosperm  a  new  plasmatic 
membrane  seems  to  be  formed  only  in  contact  with  that 
of  the  mother-cell.  In  small  embryo-sacs,  where  each 
nuclear  division  is  followed  by  a  cell-division,  the  condi- 
tions are,  evidently,  not  essentially  different  from  those  in 
vegetative  cell-division.  And,  for  those  embryo-sacs 
which  continue  to  grow  after  fructification,  I  am  not  able 
to  find,  in  the  literature  in  question,  any  proof  against  the 
correctness  of  this  assumption.76 

In  a  number  of  algae  (e.  g.,  Acetabularia,  Hydrodict- 
yon,  Ulothriv)  the  swarm-spores  arise  from  only  a  part 
of  the  protoplasm  of  the  mother-cell.  In  such  a  case  this 
part  is  always  the  peripheral  layer,  and  every  swarm- 
spore  receives,  as  far  as  the  present  literature  allows  us 
to  judge,  not  only  a  nucleus,  chromatophores,  and  vac- 
uoles,77  but  also  a  part  of  the  limiting  membrane  of  the 
mother-cell.  Similar  conditions  seem  to  exist  among  the 
fungi,  e.  g.,  in  Protomyces  macrosporus.78  In  the  case 
of  Hydrodictyon,  Pringsheim  states  that  the  colorless, 
ciliated,  anterior  end  of  the  swarm-spores  represents  the 
maternal  membrane.79  In  the  Saprolegniaceae  also,  the 

75At  this  point  in  the  original  occurs  a  discussion  of  the  pro- 
cesses of  cell-division  within  the  embryo-sac  in  their  relation  to  the 
question  of  the  autonomy  of  the  limiting  membrane.  Since  the 
points  there  considered  are  now  definitely  settled  and  agreed  upon, 
the  two  paragraphs  are  here  omitted  with  the  author's  approval.  Tr. 

76See  especially  Hegelmaier,  Zur  Entwickelungsgeschichte  endo- 
spermatischer  Gewebekorper.  Bot.  Zeit.  44:  529,  545,  561,  585.  1886. 

"According  to  the  communication  by  Went  mentioned  on  p.  154. 

78Cf.  de  Bary.  Vergleichende  Morphologic  und  Biologie  der 
Pilse,  Mycetozoen  und  Bacterien,  p.  86.  1884. 

™Monatsbericht  Kais.  Akad.  Berlin,  p.  246.      1871. 


Autonomy  of  the  Limiting  Membrane  165 

oospores  are  formed  in  such  a  way  that  each  takes  up  in 
itself  a  part  of  the  maternal  membrane.80 

We  meet  with  a  greater  difficulty  in  the  ascospores. 
But  their  origin  has  not  been  carefully  studied  in  late 
years.  Thus,  though  we  know  that  divisions  of  the 
mother-nucleus  always  precede  their  formation,  the  ques- 
tion as  to  how  they  acquire  their  other  organs  has  not 
yet  been  studied.  It  is  clear  that  every  spore  must  get  one 
or  more  vacuoles  through  the  division  of  the  maternal 
sap-vesticles,  but  how  this  comes  about,  nobody  has  yet 
investigated.  The  consideration  of  the  other  question 
also  as  to  whence  the  spores  obtain  their  plasmatic  mem- 
brane, must  be  most  urgently  recommended. 

In  the  same  way  the  origination  of  the  egg-cell  in  the 
oogonium  of  the  Peronosporales  awaits  study  by  means 
of  modern  methods.  In  this  case,  too,  nothing  definite 
can  be  said  for  the  present  in  regard  to  the  origination 
of  the  plasmatic  membrane.  Concerning  the  membrane 
of  the  spermatozoids,  consult  the  following  Section  (pp. 
174-176). 

As  a  final  result  of  this  review,  we  may  therefore  say 
that,  in  all  cases  in  which  the  arising  of  a  new  plasmatic 
membrane  is  supposed  to  take  place  without  contact  with 
the  old  one,  this  assumption  is  chiefly  due  to  investigation 
by  the  older  and  imperfect  methods.  Exceptions  to  the 
rule  are  not  at  all  known  with  certainty,  although,  accord- 
ing to  the  hypothesis  of  intracellular  pangenesis,  they 
must  not  be  considered,  a  priori,  as  impossible. 

8°De  Bary.  Abh.  Senckenb.  Naturf,  Ges.  12:  261,      1881. 


C.    THE  FUNCTIONS  OF  THE  NUCLEI 


CHAPTER  I 
FERTILIZATION 

§  /.    Historical  Introduction 

The  first  author  who  described  the  nucleus  as  the  organ 
of  heredity  was  Ernst  Haeckel.  In  the  second  volume  of 
his  "Generelle  Morphologic  der  Organismen/n  he  estab- 
lished this  conception,  founding  it  especially  on  the  be- 
havior of  the  nucleus  during  cell-division.  For  him  thfe 
"inner  nucleus  has  the  work  of  transmitting  the  hered- 
itary characters,  the  outer  plasm  has  the  part  of  adapta- 
tion, accommodation  or  adjustment  to  the  conditions  of 
the  outer  world."  And  just  as  the  nucleus  plays  its  princi- 
pal role  in  propagation,  so  is  nutrition  the  chief  task  of 
the  plasma.  In  the  lowest,  non-nucleated  organisms  the 
two  functions  are  not  yet  separated. 

For  almost  ten  years  this  prophetic  utterance  re- 
mained without  noticeable  effect  on  the  progress  of  cell- 
anatomy  and  the  theory  of  fertilization.  It  was  only 
when  Oscar  Hertwig  discovered  that  in  fertilization  the 
spermatozoids  copulate  with  the  nucleus  of  the  egg-cells 
that  Haeckel's  idea  became  the  starting-point  for  a  new 
line  of  investigation.2  Hertwig  first  observed  this  fact 
in  the  eggs  of  the  Echinidae. 

R.  Hertwig,  Fol,  Selenka,  Flemming,  and  others,  have 
lent  their  support  to  this  opinion  by  further  investigations, 

!pp.  287-289.    1886. 

2Hertwig,  O.  Beitrage  zur  Kenntnis  der  Bildung,  Befruchtung 
und  Theilung  de£  thierischen  Eies,  Morphol  Jahrb.  1:  347.  1875. 


1 70  Fertilisation 

and  in  consequence  of  this  it  is  quite  generally  recognized 
at  present  in  zoological  science. 

In  the  field  of  botany  Strasburger  has  the  merit,  by 
investigations  of  many  years'  duration,  of  having  defi- 
nitely proved  the  theory  that  fertilization  consists  essen- 
tially in  the  union  of  the  nuclei.  His  first  studies  on  the 
fertilization  of  the  conifers,  and  later  on  the  same  process 
in  the  angiosperms3  now  form  the  foundation  of  this  part 
of  our  knowledge. 

The  other  organs  of  the  protoplasts  take  no  part  in 
fertilization  during  copulation.  And  since,  in  spite  of 
this,  the  derivatives  of  the  fertilized  egg-cell  possess  later 
the  characteristics  of  both  parents,  it  is  clear  that  a  trans- 
mission to  them  of  the  hereditary  characters  from  the 
fertilized  nucleus  must  take  place.  This  transmission, 
however,  has,  at  least  so  far,  eluded  observation.  But 
many  facts,  even  outside  the  scope  of  the  theory  of  fer- 
tilization, speak  in  favor  of  its  existence. 

It  is  my  intention  to  put  together  in  this  Section,  as 
completely  as  possible,  all  the  facts  that  might  throw  any 
light  on  the  nature  of  this  transmission.  The  prevailing 
conception  regards  this  process  as  a  dynamic  one,  while 
my  hypothesis  of  intracellular  pangenesis  assumes  a 
transport  of  material  particles  as  bearers  of  the  hereditary 
characters.  Therefore  it  is  a  question  of  ascertaining 
which  of  these  two  conceptions  is  best  supported  by  the 
material  available  for  observation. 

3Strasburger,  E.  Uebcr  Befruchtung  und  Zelltheilung,  1878. 
Neue  Untersuchungen  uber  den  Befruchtungsvorgang  bei  den  Phane- 
rogamen,  1884. 


CHAPTER  II 

FERTILIZATION  (continued.) 
§  2.  The  Conjugation  of  the  Zygosporeae 

The  behavior  of  the  chlorophyll-band  of  Spirogyra 
during  conjugation  is  very  instructive.  De  Bary4  has 
already  observed  that  in  many  species  having  one  spiral 
the  two  chlorophyll-bands  of  the  conjugating  cells  join 
their  ends  in  such  a  way  that  they  form  a  continuous 
ribbon.  For  the  one-spiraled  species,  5.  Weberi,  how- 
ever, Overton  has  quite  recently  described  and  figured 
how  the  band  of  the  maternal  cell  splits  in  the  middle 
during  conjugation,  and  how  the  paternal  band  then  in- 
serts itself  between  the  two  halves  and  attaches  itself  to 
their  ends.5  Later,  owing  to  the  considerable  swelling 
of  the  pyrenoids,  as  well  as  to  other  processes,  the 
windings  of  the  band  gradually  become  more  indistinct, 
and  finally,  in  the  zygospore,  quite  indistinguishable,  un- 
til they  reappear  again  during  its  germination.6 

These  data  are  quite  sufficient  to  give  us  an  idea  of  the 
derivation  of  the  chlorophyll-bands  of  the  young  germ- 
plant.  We  assume,  as  a  result  of  the  above  mentioned 
investigations,  that  the  chlorophyll-band  of  the  germi- 
nating zygospore  consists  of  the  bands  of  the  two  sexual 
cells  which  are  joined  by  their  ends  in  one  way  or  an- 

4De  Bary.  Die  Conjugaten.  p.  3. 

5Overton,  C  E.  Ber.  Deut.  Bot.  5:  70.   Taf.  IV.      1888. 

6See  also  on  this  subject  Klebahn.   Ber.  Deut.  Bot.  Gcs.  6:  163. 


172  Fertilization 

other.7  What  will  happen  to  these  first  parts  of  the  band 
at  the  first  divisions  of  the  young  plant?  Evidently,  in 
the  case  described  by  de  Bary,  the  first  cell-division  will, 
by  cutting  the  band  through  in  the  middle,  give  the  ma- 
ternal half  to  one  daughter-cell  and  the  paternal  half  to 
the  other.  In  5.  Weberi  the  two  subsequent  divisions  will 
do  this ;  the  middle  cells  of  the  four-celled  thread  will  then 
bear  the  paternal,  the  two  end-cells,  the  maternal  band. 

The  result  of  this  speculation  is,  that,  for  the  individ- 
ual cells  of  a  one-spiraled  Spirogyra-thread,  it  makes  no 
difference  whether  they  get  their  chlorophyll-band  from 
the  father  or  from  the  mother.  However,  there  is  no 
doubt  but  that  all  the  bands  of  the  young  plant  possess, 
later,  the  same  hereditary  characters,  even  though  there 
were  individual  differences  between  father  and  mother. 
We  must  therefore  assume  that  they  necessarily  got  these 
from  the  nucleus,  after  fertilization.  If  we  attribute  to 
the  process  of  conjugation  any  significance  at  all  for  the 
active  hereditary  characters,  and  do  not  wish  to  restrict 
its  effect,  through  all  generations,  to  the  nuclei,  we  are 
evidently  compelled  to  accept  this  assumption. 

But  in  this  case  the  necessity  of  a  transmission  of  the 
hereditary  characters  from  the  fertilized  nucleus  to  the 
other  organs  of  the  protoplasts,  lies  before  us  in  a  simple 
illustration. 

We  will  generalize  this  theory,  and  say  that  in  the 
entire  plant  world  it  is  indifferent  for  the  new  individual 
whether,  with  the  exception  of  the  nucleus,  it  gets  the 
organs  of  its  protoplasts  from  the  father  or  the  mother. 

7In  other  cases  the  chlorophyll-band  of  the  male  cell  is  dis- 
organized and  resorbed.  Cf.  Chmielevsky.  V.  Eine  Notiz  iiber  das 
Verhalten  der  Chlorophyllbander  in  den  Zygoten  der  Spirogyraar- 
ten.  Bot.  Zeit.  48:  773.  1890. 


Fertilization  in  Cryptogams  173 

But  the  nucleus  must  be  from  both.  The  facts  to  be  dis- 
cussed in  the  two  following  Sections,  teach  us  that,  in 
fertilization  proper,  the  other  organs  come  from  the 
mother  only.  But  this  is  simply  to  be  regarded  as  a  spe- 
cial adaptation. 

The  chromatophores  of  the  other  Zygosporeae,  exam- 
ined with  this  end  in  view,  behave  essentially  similarly  to 
those  of  Spirogyra.  They  touch  one  another  (Eplthe- 
mia),  or  do  not  unite  (Zygnema  and  many  others),  but 
they  never  conjugate  in  the  true  sense  of  the  word.8  At 
the  first  divisions  of  the  zygospore,  the  paternal  and  ma- 
ternal chlorophyll  grains  must  therefore  always  be  dis- 
tributed to  the  individual  cells  of  the  thread. 

Schmitz,  who  was  probably  the  first  to  observe  the 
conjugation  of  the  nuclei  in  the  Zygosporeae,  and  who 
studied  carefully  the  above  mentioned  behavior  of  the 
chromatophores,  demonstrated  in  a  clear  manner  that,  in 
these  cases  also  "the  essential  point  is  only  the  union  of 
the  nucleus  of  the  male  cell  with  the  nucleus  of  the  female 
cell/'9  And  the  facts  which  have  been  discovered  later 
have  fully  confirmed  this  statement. 

§  j.  Fertilization  in  Cryptogams 

Schmitz,  in  his  important  monograph  on  the  chro- 
matophores of  the  algae,  has  comprehensively  demon- 
strated that  these  structures  which,  at  each  vegetative 
cell-division,  are  transmitted  from  the  mother-cell  to  its 
daughter-cells,  are  usually  entirely  lacking  in  the  sper- 
matozoids.10  The  egg-cells,  however,  always  possess  these 

8Schmitz.     Die  Chromatophoren  der  Algen,  p.   128.     See  also 
Overton  and  Klebahn,  loc.  cit. 
*Loc.  cit.  p.  128.  note  2. 
10Schmitz,  loc.  cit.  p.  120  ff. 


174    '  Fertilisation 

organs.  After  fertilization  they  multiply  by  division,  and 
thus  form  the  chromatophores  of  the  new  individual.  In 
regard  to  this  point  the  organization  of  the  protoplasts 
is  therefore  inherited  directly  from  the  mother  and  not 
from  the  father. 

Let  us  now  see,  how  the  other  members  of  the  proto- 
plast, with  the  exception  of  the  nucleus,  behave.  To  all 
appearances  the  spermatozoids  possess  neither  vacuoles 
nor  chromatic  bodies,  and  hence  the  condition  is  the  same 
for  the  former  as  for  the  latter. 

According  to  the  best  recent  investigations,  the  sper- 
matozoids do  not  originate,  as  some  authors  previously 
assumed,  from  the  nucleus  only  of  the  mother-cell,  but 
the  rest  of  the  plasma  also  takes  part  in  their  formation. 
It  is  true  that  the  nucleus  forms  the  bulk  of  the  body  of 
the  male  reproductive  cell.  Schacht  has  already  voiced 
the  theory,  on  the  basis  of  his  observations  and  those  of 
others,  "that  the  nucleus  takes  a  very  active  part  in  the  for- 
mation of  the  spermatozoid  and  in  a  certain  way  blends 
into  it."11  He  declares  further  that,  in  this  process,  the 
granular  contents  of  the  mother-cell  disappear.  This  trans- 
formation of  the  nucleus,  although  denied  by  prominent 
investigators12  at  the  beginning  of  the  more  recent  re- 
searches, is  now  generally  recognized  as  the  most  im- 
portant part  of  the  whole  process. 

Outside  the  nucleus  there  lies,  in  the  spermatozoids, 
the  limiting  membrane,  which  protects  this  organ  against 
external  influences,  and,  in  a  certain  way,  serves  as  the 
little  boat  that  carries  it  to  its  destination.  The  distinc- 

"Schacht.  Die  Spermatozoiden  p.  35.      1864. 

12Comp.  e.  g.  Sachs,  Lehrbuch,  4.  Auflage,  p.  303;  and  Stras- 
burger,  Zellbildung  und  Zelltheilung,  III  Aufl.  p.  94;  also  Bot.  Zeit. 
39:  847,  848.  1881. 


Fertilisation  in  Cryptogams  175 

tion  of  these  two  parts  we  owe  chiefly  to  Zacharias,  who 
thoroughly  investigated  the  micro-chemical  reactions  of 
the  male  reproductive  cells,  and  pointed  out  repeatedly 
the  different  behavior  of  their  external  and  internal 
parts.13  The  nuclein  especially  forms  the  chemical  char- 
acteristic for  the  substance  of  the  nuclei.  Fluids  which 
easily  dissolve  and  extract  this  substance  remove  only  the 
inner  part  of  the  spermatozoids  and  leave  the  outer  layer 
and  the  cilia  in  general  undissolved.  In  return  the  cilia 
dissolve  in  pepsin,  and  do  not,  therefore,  consist  of  nu- 
clein.1* According  to  Campbell,  also,  the  cilia  of  the  sper- 
matozoids are  not  developed  from  the  nucleus,  but  from 
the  cytoplasm  of  the  mother-cell.15 

But,  during  fertilization  evidently  the  nucleus  alone 
plays  a  part.  The  deep  penetration  of  the  entire  sper- 
matozoid  into  the  egg-cells  teaches  that  there  is  no  prob- 
ability of  a  conjugation  of  its  outer  layer  with  that  of  the 
egg-cell.  More  likely  do  this  organ  and  the  cilia  dis- 
appear within  the  egg-cell,  without  playing  any  note- 
worthy role  therein. 

Exceptionally  the  spermatozoids  possess  small  chromat- 
ophores  which,  perhaps,  they  may  need  on  the  way  to  the 
egg-cell,  either  for  taking  the  right  direction,  or  for  other 
purposes.  An  example  is  found  in  Fucus,  where  Schmitz 
proved  that  they  arise  by  division  from  the  chromato- 
phores  of  the  mother-cell.16  But  no  observation  teaches 
that  they  play  any  role  in  fertilization. 

Phylogenetically,  the  spermatozoids  of  the  algae  have 

isZacharias.  Bot.  Zeit.      1881-1888. 

14Zacharias,  E.  Ueber  die  Spermatozoiden.  Bot.  Zeit.  39:  828, 
836, 850.  1881. 

15Campbell,  D.  H.  Zur  Entwickelungsgeschichte  der  Spermato- 
zoiden. Ber.  Deut.  Bot.  Ges.  5:  120.  1887. 

16Schmitz,  loc.  cit.  p.  122. 


1 76  Fertilization 

doubtless  originated  from  conjugating  swarm-spores.  In 
time  they  have  gradually  lost  their  chromatic  bodies,  and 
probably  also  their  vacuoles.  For  the  disappearance  of 
the  former  Schmitz  describes  a  number  of  intermediate 
steps.  May  I  be  allowed  to  quote  the  following  sentences 
from  his  important  treatment  of  this  subject:17  "Some- 
times, especially  where  the  difference  of  the  two  kinds  of 
sexual  cells  is  not  yet  very  considerable,  the  spermato- 
zoids  act  exactly  like  the  isogametes,  and  like  these 
retain  the  chromatophores  unchanged  (e.  g.,  in  Scyto- 
siphon  lomentarium) .  As  that  difference  becomes  greater, 
however,  the  chromatophores  of  the  male  cells  show  a 
distinct  tendency  to  disappear,  and  especially  does  their 
coloring  become  less  intense  (Bryopsis)" 

This  comparative  study  bridges  the  chasm  lying  be- 
tween conjugation  and  fertilization,  which  is  no  doubt 
chiefly  due  to  the  fact  that,  in  the  latter,  the  organization 
of  the  protoplasts  is  inherited  morphologically  from  the 
mother  only,  while  in  the  former,  in  some  cells,  the  in- 
heritance is  from  the  mother,  in  others  from  the  father. 
But,  on  the  other  hand,  the  above  mentioned  phylogenetic 
consideration  leads  to  the  conviction  that  the  outer  layer 
of  the  spermatozoids  has  the  same  significance  and  the 
same  origin  as  that  of  the  swarm-spores,  and  is  just  as  in- 
dispensable. 

§  4.  Fertilization  in  Phanerogams 

In  the  seed-bearing  plants,  also,  the  organization  of 
the  protoplasts  is  directly  inherited  from  the  egg-cell 
alone.  From  the  pollen-tube  only  the  nucleus  penetrates 
into  the  latter;  other  parts,  even  if  they  should  be  neces- 
sary for  the  transportation  of  the  nucleus  and  should  ac- 
cit.  p.  121. 


Fertilization  in  Phanerogams  177 

company  it,  do  not  play  any  role  in  the  true  process  of 
fertilization. 

Everybody  is  acquainted  with  the  valuable  investiga- 
tions of  Strasburger  in  this  field  which,  since  1878,  have 
repeatedly  treated  this  point  and  have  completely  proven 
the  above  mentioned  theories.  It  would  be  superfluous 
to  redescribe  them  here,  or  to  enumerate  their  confirma- 
tions by  other  investigators. 

How  the  nuclei  unite  during  fertilization  is  a  question 
which  is  very  far  from  haying  been  satisfactorily  an- 
swered. Furthermore,  differences  predominate  here 
which  are  at  least  very  striking.  According  to  Stras- 
burger, not  only  do  the  nuclear  skeins  fuse,  but  also  the 
nuclear  vacuoles,  and  hence  the  nuclear  sap.18  Accord- 
ing to  van  Beneden,  the  nuclear  skeins  of  the  male 
and  the  female  cells  in  Ascaris  megalocephala  arrange 
themselves  side  by  side  and  form  the  segmentation  nu- 
cleus.19 They  seem  to  unite  at  their  ends,  thus  forming  a 
single  nuclear  thread,  in  which,  therefore,  only  juxtapo- 
sition takes  place,  and  not  a  mutual  penetration  of  their 
elements.  But  while,  in  animals,  according  to  the  avail- 
able data,  fusion  does  take  place  during  the  state  when 
the  chromosomes  are  arranged  in  the  form  of  a  star,  it  is 
seen  to  occur  in  the  plants  in  the  state  of  rest.  Whether 
this  difference  really  exists,  and  how  the  nuclear  threads 
generally  unite,  are  questions  which  have  to  be  more 
thoroughly  investigated.20 

It  is  significant  that  the  number  of  the  chromosomes, 
according  to  Strasburger's  most  recent  investigations,  has 

l8Strasburger.     Ueber  Kern-   und  Zelltheilung,   p.   230.    Jena. 


19Van  Beneden,  E.     Recherches  sur  la  maturation  de  I'  oeuf. 
1883. 

29Strasburger.  Ueber  Kern-  und  Zelltheilung.  p.  240.  Jena.      1888. 


178  Fertilisation 

also  been  found  to  be  constant  in  plants  in  the  generative- 
cells  of  every  species,  being  the  same  for  the  male  cells 
as  for  the  female.  Sometimes  it  is  the  same  for  large 
groups  of  plants  as,  e.  g.,  for  the  Orchidacese  16;  in  the 
Liliacese  it  varies21  between  8,  12,  16  and  24.  For  Ascaris 
megalocephala  it  is  2,  for  A.  lumbricoides  24.  Obviously 
this  number  does  not  have  any  systematic  significance  or 
stand  in  any  relation  to  the  hereditary  characters. 

However,  from  a  continued  investigation  in  this  field, 
we  may  expect  important  disclosures  on  the  question  as 
to  which  parts  of  the  nucleus  are  the  real  bearers  of  the 
latent  hereditary  characters.  For  the  present  the  evi- 
dence is  in  favor  of  the  assumption  that  they  are  to  be 
looked  for  in  the  chromosomes.22  For  the  further  work- 
ing out  of  the  theory  of  heredity  this  is,  without  doubt, 
of  the  highest  interest;  for  our  hypothesis,  however,  a 
decision  is  not  absolutely  necessary. 

siStrasburger.    Loc.  tit.  pp.  239,  242. 

22Roux,  Ueber  die  Bedeutung  der  Kernfiguren,  1883. 


CHAPTER  III 

THE    TRANSMISSION    OF    HEREDITARY    CHARACTERS 

FROM  THE  NUCLEI  TO  THE  OTHER  ORGANS 

OF  THE  PROTOPLASTS 

§  5.    The  Hypothesis  of  Transmission 

The  question  of  a  transmission  of  hereditary  charac- 
ters from  the  nuclei  to  the  other  organs  of  the  protoplasts 
has  been  repeatedly  raised  in  the  foregoing  sections.  But, 
if  we  review  all  the  facts  combined  in  the  preceding  chap- 
ter, and  in  this,  the  necessity  of  the  assumption  of  such 
transmission  is  forced  upon  us. 

The  protoplasts  of  the  plant  possess  a  visible  organi- 
zation, which,  at  every  cell-division,  is  transmitted  by 
division  of  the  individual  organs,  directly  from  the 
mother-cell  to  its  daughter-cells.  The  heredity  is  here 
a  visible  and  not  a  latent  one.  But  the  individual  or- 
gans are  ontogenetically  independent  from  each  other; 
they  originate  only  through  the  division  of  such  as  are 
already  present.  And  even  if,  in  the  course  of  develop- 
ment, they  adapt  themselves  to  various  functions  and,  in 
doing  so,  receive  other  names,  and  although  their  origin 
in  individual  cases  is  not  yet  cleared  up,  so  much  is,  on 
the  whole,  certain,  that  the  nucleus,  the  chromatophores, 
the  vacuoles  and  the  granular  plasm,  and  probably  also 
the  limiting  membrane,  are  primary  organs  which  never 
arise  from  each  other,  but  only  multiply  side  by  side. 

Each  of  these  primary  organs  possesses  a  complement 
of  characters  and  potentialities  which,  together,  form  the 


180        Intracellular  Transmission  of  Characters 

character  of  the  species.  These  qualities  can  either  be 
seen  directly  under  the  microscope,  or  they  betray  their 
presence  by  definite  functions.  That  the  hereditary  char- 
acters lie  in  the  respective  organs  of  the  protoplasts  can 
hardly  be  doubted.  But  whether  they  also  lie  thus  in  cells 
where  they  are  present  only  in  the  latent  condition  is  not 
disclosed  by  the  processes  of  vegetative  propagation. 

Here  the  process  of  fertilization  serves  as  a  clue.  Hy- 
brids teach,  and  daily  observations  on  man  confirm  the 
fact  that  children,  on  an  average,  receive  their  character- 
istics, to  the  same  extent,  from  both  parents.  But  the 
fertilized  egg-cell  receives  its  organs  from  the  mother 
only,  while  from  the  father  only  the  sperm-nucleus 
conjugates  with  the  nucleus  of  the  egg-cell.  All  the 
hereditary  characters  of  the  father  must  therefore 
be  transmitted  in  the  nucleus,  as  potentialities  in  a 
latent  state.  And  before  they  can  become  active  in  the 
other  organs  of  the  protoplast,  they  must  evidently  be 
transported  to  the  latter  ones  from  the  nucleus.  This 
transmission  is  therefore  a  hypothesis,  the  assumption  of 
which  may  well  be  regarded  as  a  necessity  at  the  present 
state  of  our  knowledge. 

May  I  be  allowed  to  illustrate  this  transmission  by  a 
few  examples.  I  take  them  from  hybrids,  because  here 
the  relations  lie  most  clearly  and  convincingly  before  us, 
and  I  chose  the  colors  of  the  flowers  bec?use  they  are 
easily  observed. 

Let  us  first  take  the  red  color  of  flowers.  Phase o- 
lus  multiflorous  has  red  flowers,  Phaseolus  vulgaris  nanus 
white  ones.  By  pollinating  the  latter  with  the  pollen  of 
the  former  there  came  about  several  times,  in  1886,  in  my 
own  cultures, -a  hybrid  seed.  This  does  not  deviate  ex- 
ternally from  the  normal  seed  of  its  mother-plant,  but  it 


The  Hypothesis  of  Transmission  181 

develops  into  a  plant  which  is  similar  to  the  twining  P. 
multiflorous,  but  remains  smaller  than  the  latter.  The 
flowers  of  the  hybrid  are  of  a  pale  red,  being  a  tint  midway 
between  the  two  parents,  as  I  had  the  opportunity  of 
convincing  myself  personally.  The  red  coloring  matter 
is  found  in  solution  in  the  vacuoles  of  the  cells  of  the 
petals. 

The  ability  of  the  vacuoles  to  form  the  red  erythro- 
phyll  comes  from  the  father,  in  this  instance.  But  the 
vacuoles  of  the  hybrid  originate  morphologically  from 
those  of  the  mother.  The  power  of  producing  erythro- 
phyll  must  therefore  have  been  transmitted  in  a  latent 
condition  in  the  sperm-nucleus  of  the  father  to  the  nu- 
cleus of  the  egg-cell,  and  must  have  been  communicated 
sooner  or  later  to  the  vacuoles  of  the  hybrid. 

The  same  thing  is  taught  by  many  other  hybrids,  as, 
for  example,  Digitalis  lutea  9  x  purpurea  $ ,  Linaria 
vulgaris  9  x  purpurea  $ ,  Linaria  genistaefolia  ?  x  pur- 
purea $ ,  et  cetera.2* 

The  yellow  color  of  the  flowrers  behaves  in  the  same 
way.  Digitalis  lutea-purpurea  forms  the  best  illustra- 
tion. The  two  forms  D.  purpurea  $  x  lutea  $  and  D. 
lutea  $  x  purpurea  $  are  quite  alike,  with  the  exception 
of  some  slight  variations  in  the  color  of  the  flowers.24 
Naudin  gives  an  illustration  of  the  hybrid ;  the  flower  has 
a  pure  yellow  color  in  one  cluster,  while  in  the  other  one, 
yellow  is  mixed  with  pale  red.25  Of  the  two  mentioned 
hybrids  of  the  Linaria  I  do  not  find  any  record  of  the 
reciprocal  forms. 

23Cf.  Focke,  Die  Pflansenmischlinge,  pp.  311,  315,  and  other 
passages. 

24Focke,  loc.  cit.  p.  315. 

25Naudin.  Nouvelles  recherches  sur  1'hybridite.  Nouvelles  Ar- 
chives du  Museum  d'histore  naturelle  de  Paris,  p.  95,  PI.  2.  1869. 


182        Intracellular  Transmission  of  Characters 

Like  the  qualities  of  the  vacuoles,  those  of  the  chro- 
matophores  must  be  communicated  to  the  hybrid  during 
hybridization,  in  a  latent  condition  in  the  pollen-nucleus 
of  the  father.  As  an  instance  I  mention  Raphanus  sativus 
9  X  Brassica  oleracea  $  ,  Medicago  sativa  $  x  falcata  $  , 
Geum  album  9  x  urbamim  $ ,  Verbascum  phoeniceum  9 
x  blattaria  $  .2Q 

Similar  instances  can  be  found  in  great  number  in  the 
abundant  literature  on  hybridization-experiments.  But 
science  greatly  needs  a  comprehensive  miscroscopic  study 
of  hybrids  in  relation  to  the  anatomical  structure  of  their 
parents.27 

Still  more  forcibly  and  more  generally  do  we  feel  the 
necessity  for  the  assumption  of  a  transmission,  when  we 
observe  the  hybrids  in  the  second  and  following  genera- 
tions. Almost  always,  when  cultivated  in  a  sufficiently 
great  number,  some  of  them  revert  to  the  grand-mother, 
others  to  the  grand- father.  The  latter  ones  are  so  similar 
that  they  could  be  easily  confounded  with  the  grand- 
father. This  teaches  us  that  in  hybridization,  all  the 
characters  of  the  father  pass  on  to  the  hybrid,  where  they 
are  present  in  the  latent  state  only,  but  that  they  become  ac- 
tive again  in  some  of  its  children.  All  the  organs  of  the 
protoplasts  must  therefore  be  able  to  draw  their  active 
characters  from  the  nucleus. 

In  the  hybrid,  however,  the  characters  of  father  and 
mother  are  equally  represented.  Especially  are  both  hy- 

26These  instances  are  from  Focke,  where  more  can  easily  be 
found.  I  regret  to  say  that  I  had  no  opportunity  of  controlling  the 
nature  of  the  yellow  coloring  matter. 

27The  "Comparison  of  the  Minute  Structure  of  Plant  Hybrids 
with  that  of  their  Parents,  and  its  Bearing  on  Biological  Problems," 
by  J.  M.  MacFarlane  (Trans.  Roy.  Soc.  Edinburgh,  37:  203.  1892)  is 
still  practically  the  only  investigation  in  this  field.  Tr. 


The  Influence  of  the  Nucleus  in  the  Cell         183 

brids  produced  by  two  species,  in  which  the  one  species 
will  function  at  one  time  as  the  father  and  at  another 
time  as  the  mother,  with  few  exceptions,  essentially  alike. 
There  is  no  ground  for  the  assumption  that  the  hereditary 
characters,  latent  in  the  egg-cell  and  in  the  spermatozoid, 
are  inherited  in  a  fundamentally  different  manner  from 
the  father  than  from  the  mother.  And  thus  we  arrive  at 
the  conclusion  that  the  latter,  too,  must  lie  in  the  nu- 
cleus, and  are  not  distributed  over  the  individual  organs 
of  the  egg-cell. 

Hence  the  nuclei  are  the  bearers  of  the  latent  hered- 
itary characters.  In  order  to  become  active,  the  greater 
part  of  these  characters,28  at  least,  must  pass  from  the 
nuclei  into  the  other  organs  of  the  protoplasts 

§  6.    Observations  on  the  Influence  of  the  Nucleus  in  the 

Cell 

Even  the  first  investigators  of  this  organ  realized 
that  the  nucleus  plays  a  prominent  role  in  the  life  of  the 
cell.  They  have  given  expression  to  this  conviction  in  the 
name  itself.  And,  although  later  the  supposed  absence  of 
the  nucleus  in  large  groups  among  the  Thallophytes  gave 
rise  to  a  doubt  as  to  the  correctness  of  this  opinion,29  it 
has  been  entirely  removed  by  more  recent  investigations. 

At  first  it  was  impossible  to  form  any  idea  as  to  the 
nature  of  that  role.  The  investigators  mentioned  in  the 
first  chapter  of  this  Section,  Haeckel,  Hertwig,  Flem- 
ming,  Strasburger,  and  others,  were  the  first  to  teach  us 
to  regard  the  nucleus  as  the  real  organ  of  heredity. 
And  even  in  these  later  years  there  are  some  authors  who 

28The  characters  that  regulate  nuclear  division,  are  probably 
active  in  the  nuclei  themselves. 

29Cf.  Brucke,  Sitzungsber.  Akad.  Wiss.  Wien.  1861. 


184        Intracellular  Transmission  of  Characters 

still,  in  opposition  to  Haeckel's  positive  assurance,  re- 
gard the  nucleus  as  an  organ  of  nutrition,  ascribing  to  it 
an  influence  on  the  formation  of  protein,  starch,  or  other 
products  of  assimilation. 

Owing  to  the  influence  of  the  above  named  investi- 
gators, attention  has  been  directed,  in  recent  years,  more 
and  more  to  the  nucleus.  In  consequence  of  this,  a  series 
of  observations  have  been  made  and  published,  which 
speak  in  favor  of  the  fact  that  the  nucleus,  although  not 
self -active,  still  exercises  a  very  great  influence  on  the 
most  important  processes  in  cell-life.  On  the  whole,  the 
conditions  observed  must,  without  doubt,  be  reduced  to 
this,  that  the  hereditary  characters,  as  long  as  they  are 
latent,  are  stored  up  in  the  nucleus,  and  become  active 
only  in  the  other  organs  of  the  protoplasts.  But  it  must 
not  be  forgotten  that,  in  individual  cases,  there  may  be  a 
special  correlation  between  nucleus  and  protoplasm,  which 
must  be  attributed  to  specific  adaptations,  and  not  to 
general  laws.  In  the  individual  case  it  will  usually  be 
very  difficult  to  decide  between  these  two  possibilities. 

First,  I  shall  describe  some  of  the  conditions  empha- 
sized already  by  the  older  investigators.  In  young  cells 
the  nucleus  lies  in  the  middle  of  the  cell.  With  the  in- 
creasing size  of  the  vacuoles,  when  the  protoplasm  reaches 
the  so-called  foamy  state,  it  remains  in  that  position  and 
is  connected  with  all  the  parts  of  the  peripheral  plasm  by 
bands  and  strands  radiating  from  it  by  the  shortest  lines. 
This  familiar  picture,  and  the  considerable  size  of  the  nu- 
cleus in  young  cells,  may  have  been  the  first  reasons  for 
attributing  special  importance  to  this  organ.  The  nucleus 
does  not  grow  correspondingly  with  the  increasing 
growth  of  the  cells.  It  becomes  relatively  smaller,  and 
the  fusion  of  the  vacuoles  forces  it  out  of  its  central  posi- 


The  Influence  of  the  Nucleus  in  the  Cell         185 

tion.  Ordinarily,  it  does  not  take  any  definite  position 
after  this,  but  is  moved  around  in  the  cell  by  the  cur- 
rents of  the  granular  plasm.  As  Hanstein  describes  it, 
the  nucleus  traverses  a  long  and  very  tortuous  way  within 
a  few  hours,  and  sails  in  all  directions  throughout  its 
whole  domain,  "as  if  to  inspect  it  everywhere."30  Every- 
thing argues  for  the  assumption  that  the  activity  of  the 
entire  protoplast  is  under  the  regulating  influence  of  the 
nucleus.31 

Besides  the  general  behavior  of  the  nuclei  the  in- 
vestigations of  Tangl,  Haberlandt,  Korschelt,  and  others, 
have  made  us  acquainted  in  recent  years  with  a  special 
relation  of  the  nuclei  to  individual  processes  in  cell-life. 

Tangl  observed  bulb-scales  of  A  Ilium  Cepa,  which  had 
been  recently  wounded,  for  example,  the  day  before.32  He 
saw  that  near  the  wound-surface  the  nuclei  are  not,  as 
otherwise,  irregularly  distributed  over  the  cells,  but  that 
they  had  gone  to  that  side  of  their  cells  which  was  nearest 
to  the  wound.  With  them  the  granular  plasm  was  also 
accumulated  on  those  walls.  The  shorter  the  distance 
from  the  wound,  the  more  pronounced  was  the  phenom- 
enon, but  as  far  away  as  about  0.5  mm.  it  could  still 
be  distinctly  seen.  These  conditions  probably  indicate 
that  the  process  of  regeneration  which  the  wounds  usually 
cause  proceed  here,  under  the  influence  of  the  nuclei. 

Haberlandt  studied  the  position  of  the  nucleus  during 
this  process  in  a  great  number  of  cases  in  which  the  cells 
of  the  higher  plants  show  a  more  vigorous  local  growth 

30Hanstein,  Das  Protoplasma.  1:  165.     1880. 

31Cf.  Strasburger.   Neue  Untersuchungen.  p.  125.     1884. 

32Tangl,  E.  Zur  Lehre  von  der  Continuitat  des  Protoplasmas 
im  Pflanzengewebe.  Sitzb.  Math.-Naturw.  CL  Akad.  Wiss.  Wien. 
90:  10.  1884. 


186        Intracellular  Transmission  of  Characters 

in  some  definite  part  of  their  circumferences.33  He  did 
so  partly  where,  through  localized  surface  growth,  the 
shape  of  the  cells  changes,  partly  where  unilateral  thick- 
enings of  the  membranes,  or  a  definite  wall  sculpture  are 
started.  And  although,  owing  to  the  abundance  of  in- 
dividual phenomena,  a  rule  without  exceptions  could  not 
be  expected,  he  found,  on  the  whole,  that  the  nucleus  most 
frequently  turns  to  where  growth  is  strongest,  and  re- 
mains longest  where  the  latter  continues  longest. 

According  to  Korschelt,  the  same  rule  is  valid,  in  a 
general  way,  for  the  animal  cell.34  With  chiefly  unilateral 
or  local  activity  of  the  cells,  this  investigator  succeeded, 
in  a  number  of  cases,  in  observing  for  the  nucleus  a  defi- 
nite position  which  was  as  near  as  possible  to  the  place 
where  this  process  was  going  on.  Frequently,  when  the 
distance  is  more  considerable,  the  nucleus  is  connected 
with  such  favored  places  by  bands  and  accumulations  of 
protoplasm. 

Where  the  nucleus  does  not  betray  its  influence  on 
the  processes  in  the  protoplasm  by  a  change  of  position, 
it  does  so  frequently  by  a  definite  arrangement  of  the 
latter  around  the  nucleus.  The  accumulation  of  the  amy- 
loplasts  in  the  immediate  vicinity  of  the  nucleus,  as  is 
frequently  observed  in  young  cells,  has  been  ascribed  by 
various  investigators  to  the  influence  of  the  nucleus  on 
their  activity.35  Pringsheim  has  demonstrated  that,  in 

33Haberlandt,  G.  Ueber  die  Beziehungen  zwischen  Funktion  und 
Lage  des  Zellkernes.  1887. 

34Korschelt,  E.  G.  Haberlandt,  Ueber  die  Beziehungen  zwischen 
Funktion  und  Lage  des  Zellkerns  bei  Pflanzen,  Jena,  1887,  nebst 
einigen  Mitteilungen.  Biol.  Cent.  8:  110.  1888. 

85Cf.  e.  g.  Strasburger,  Ueber  Kern  und  Zelltheilung,  p.  195. 
1888.  Schimper,  A.F.W.  Untersuchungen  uber  die  Chlorophyll- 
korper,  und  die  ihnen  homologen  Gebilde.  Jahrb.  Wiss.  Bot.  16: 
1.  1885.  Haberlandt,  G.  Die  Chlorophyllkorper  der  Selaginellen. 
Flora.  71:291.  1888. 


The  Influence  of  the  Nucleus  in  the  Cell         187 

the  cells  of  Spirogyra,  the  threads  which  radiate  from 
the  nuclear  cavity  attach  themselves  especially  to  the 
pyrenoids  of  the  chlorophyll  bands,  and  by  ramifying, 
frequently  connect  several  of  them  directly  with  the  nu- 
cleus.36 In  cell-formation  in  those  embryo-sacs  where 
the  new  cells  arise  in  a  peripheral  layer,  after  the  forma- 
tion of  numerous  nuclei,  Strasburger  has  repeatedly  de- 
scribed radiated  figures  which  unite  the  nuclei,  and  which 
are  present,  not  only  between  the  two  daughter-cells  of 
a  mother-cell,  but  also  are  placed  between  the  nuclei  that 
are  not  so  closely  related  to  each  other.  The  repeated 
studies  of  this  investigator  certainly  remove  all  doubt 
of  the  fact  that  along  these  rays  some  influence  from  the 
nuclei  makes  itself  felt  during  cell-division.37  _. 

The  multinuclear  nature  of  the  coeloblasts,  discovered 
and  carefully  studied  especially  by  Schmitz,38  also  argues 
for  the  great  importance  of  the  nucleus.  As  a  rule,  here 
the  nuclei  do  not  lie  in  the  moving  part  of  the  granular 
plasm,  but  in  its  resting  layers.  They  are  arranged 
evenly  at  almost  equal  distances  from  each  other,  and 
are  mostly  small  and  so  numerous,  that  every  detached 
piece,  if  indeed  not  too  small  to  remain  alive,  probably 
always  contains  one  or  more  nuclei.  All  parts  of  the 
protoplasts  can  evidently  be  directly  influenced  by  them. 

Following  the  observations  on  uninjured  cells,  the 
investigations  on  injured  protoplasts  must  lastly  be  dis- 
cussed. Schmitz  has  already  drawn  attention  to  the  fact 
that  the  extruded  protoplasmic  balls  of  Vaucheria  and 
other  Siphonocladiaceae,  are  enabled  to  form  a  new  cell- 

86Pringsheim,  N.  Ueber  Lichtwirkimg  und  Chlorophyll  Function 
in  der  Pflanze.  Jahrb.  Wiss.  Bot.  12:  304.  1881. 

37Cf.  e.  g.  Strasburger,  E.  Bot.  Praktikum,  1  Aufl.  p.  610. 

38 Schmitz.  Die  vielkernigen  Zellen  der  Siphonocladiaceen.  Fest- 
schr.  Naturf.  Ges.  Halle.  1879. 


OF   THE 

UNIVERSITY   ) 

OF 

.;.^ 


188        Intracellular  Transmission  of  Characters 

membrane  and  to  regenerate  into  new  vital  individuals 
only  when  they  possess  one  or  several  nuclei.39  This 
must  not  be  understood  to  mean  that  the  nucleus  is  the 
only  condition.  The  chromatophores  and  the  other  or- 
gans of  the  other  protoplasts  must  also  be  present,  but 
the  significance  of  these  for  growth  and  nutrition  is  of 
such  a  nature  that  their  indispensability  may  be  regarded 
as  a  matter  of  course.  Nussbaum  and  Gruber  have  later 
proven  through  extensive  experiments  in  the  division  of 
protozoa,  that  here  too  the  fractional  parts  of  the  proto- 
plasts can  regenerate  completely  only  when  the  nucleus, 
at  least,  is  not  lacking.40 

The  experiments  of  Klebs  on  the  culture  of  plas- 
molysed  cells  are  also  important.41  I  take  from  them 
what  follows :  If  cells  of  Zygnema  and  Oedogonium  are 
plasmolysed  in  a  10%  solution  of  glucose,  the  contents 
of  the  longer  cells  not  infrequently  divide  into  two  or 
more  pieces,  which,  joined  at  first  by  thin  threads,  later 
separate  entirely  from  each  other.  If  the  threads  are 
now  grown  in  light  in  this  solution,  the  contracted  pro- 
toplasts surround  themselves  with  a  new  cell-wall,  which 
gradually  increases  in  thickness.  Sooner  or  later  they 
begin  to  grow  and  divide,  and  in  so  doing,  break  through 
the  old  cell-membrane.  But  in  those  cells  where  the 
contents  are  split  into  two  or  more  parts,  of  which,  of 
course,  only  one  can  get  the  nucleus,  only  this  latter  part 
forms  a  new  cell  membrane;  the  non-nucleated  pieces 

S9Loc.  dt.  p.  34. 

40Nussbaum,  Ueber  die  Theilbarkeit  der  lebenden  Materie, 
Archiv  Mikr.  Anatomic.  1886.  Gruber,  A.  Ueber  Kunstliche  Thei- 
lung  bei  Infusorien.  Biol  Cent.  4:  717.  1885;  Ber.  Naturf.  Ges., 
Freiburg  i-B.  1886. 

41Klebs,  G.  Ueber  das  Wachsthum  Plasmolysirter  Zellen.  Bot. 
Cent.  28:  156.  1886;  Arbeiten  Bot.  Instituts.  Tubingen.  2:  565.  1888. 


The  Influence  of  the  Nucleus  in  the  Cell         189 

can,  it  is  true,  produce  starch  and  nourish  themselves, 
but  they  are  not  able  to  grow. 

In  order  to  get  more  information  on  the  role  of  the 
nucleus  a  method  would  evidently  be  needed,  which  would 
allow  us  to  kill  the  nucleus  without  injuring  the  cell  body. 
Perhaps  this  end  could  be  attained  by  making  use  of  the 
method  suggested  by  Pringsheim,  of  partially  killing  the 
cells  in  the  focal  point  of  a  lens.42  By  selecting  a  lens 
that  makes  it  possible  to  strike  a  single  point  of  the  cell, 
it  could  be  focused  on  the  nucleus  with  a  dim  light,  and 
then  a  brief  exposure  to  the  direct  rays  of  the  sun  might 
produce  the  desired  result  in  some  of  the  cells.  I  there- 
fore warmly  recommend  this  method  for  further  elabo- 
ration in  this  direction. 

In  reviewing  the  results  of  the  investigations  that 
have  been  discussed,  we  see  that  the  nuclei  have  an  in- 
fluence on  the  activity  of  the  other  members  of  the  proto- 
plast. They  exercise  this  influence  only  as  long  as  the 
respective  members  remain  in  the  most  intimate  proto- 
plasmic connection  with  them,  preferably  at  the  shortest 
possible  distance,  or  otherwise  by  direct  plasma-bands. 

42Pringsheim,  N.  Jahrb.  Wiss.  Bot.  12:  331.    1881. 


D.     THE  HYPOTHESIS  OF  INTRACELLULAR 

PANGENESIS 


CHAPTER  I 

PANGENS  IN  THE  NUCLEUS  AND  CYTOPLASM 
§  i.     Introduction 

We  shall  now  try  to  connect  with  each  other  the 
conclusions  to  which  the  critical  survey  of  previous  the- 
ories of  heredity,  in  the  first  Part,  and  the  review  of  the 
present  state  of  the  cell  theory,  in  the  second  Part,  have 
lead  us. 

The  result  of  the  first  Part  was  that  the  comparative 
consideration  of  the  world  of  organisms,  from  the  broad- 
est standpoint,  compels  us  to  regard  specific  characters  as 
being  composed  of  innumerable,  more  or  less  independ- 
ent factors,  of  which  by  far  the  most  recur  in  various, 
and  many  in  extremely  numerous  species.  The  almost 
unbounded  variety  of  living  and  extinct  organisms  is 
thus  reduced  to  the  numerous  different  combinations 
which  a  comparatively  small  number  of  factors  makes 
possible.  These  factors  are  the  individual  hereditary 
characters,  which,  indeed,  most  frequently,  are  ex- 
tremely difficult  to  recognize  as  such  in  the  intricate  sum 
total  of  the  phenomena,  but  which,  however,  since  every 
one  of  them  can  vary  independently  from  the  others, 
may,  in  many  cases,  be  subjected  separately  to  experi- 
mental treatment. 

These  hereditary  characters  must  be  groundedjnjiy- 
ing  matter;  every  vegetative  germ-cell,  every  fertilized 
egg-cell  must  potentially  contain  within  itself  all  the  fac- 
tors that  go  to  make  up  the  characters  of  the  respective 


194         Pang  ens  in  the  Nucleus  and  Cytoplasm 

species.  The  visible  phenomena  of  heredity  are  hence 
the  expressions  of  the  characters  of  minutest  invisible 
particles,  concealed  in  that  living  matter.  And  we  must, 
indeed,  in  order  to  be  able  to  account  for  all  the  phenom- 
ena, assume  special  particles  for  every  hereditary  char- 
acter. I  designate  these  units,  pangens. 

These  pangens,  invisibly  small,  yet  of  quite  another 
order  than  the  chemical  molecules,  and  each  of  them  com- 
posed of  innumerable  such  molecules,  must  grow  and 
multiply,  and  must  be  capable  of  distributing  themselves 
by  means  of  ordinary  cell-division,  over  all  or  at  least 
nearly  all  cells  of  the  organism.  They  are  either  inac- 
tive (latent),  or  active,  but  they  can  multiply  in  both 
states.  Predominantly  inactive  in  the  cells  of  the  germ- 
tracks,  they  usually  develop  their  highest  activity  in  the 
somatic  cells.  And  this  in  such  a  way,  that,  in  higher 
organisms,  not  all  the  pangens  of  any  given  cell  probably 
ever  become  active,  but  in  every  cell  one  or  more  of  the 
groups  of  pangens  dominates  and  impresses  its  character 
on  the  cell. 

Fertilization  consists  in  a  fusion  of  nuclei.  The 
offspring  receives  from  the  father  only  that  which  was 
contained  in  the  nucleus  of  the  sperm.  All  the  hereditary 
characters  must  therefore  be  represented  in  the  nuclei 
by  their  respective  pangens.  Nuclei,  therefore,  are  to 
be  regarded  as  the  reservoirs  of  hereditary  characters. 

In  the  nucleus,  however,  by  far  the  most  of  the  char- 
acters remain  latent  all  through  life.  They  become  active 
only  in  the  other  organs  of  the  protoplast.  Haeckel  has 
already  said  "that  the  nucleus  within  had  to  take  care  of 
the  transmission  of  the  hereditary  characters,  and  the 
surrounding  plasm,  of  the  adjustmment,  accommodation, 
or  adaptation  to  environmental  conditions."  (Cf.  p.  169). 


All  Protoplasm  Composed  of  Pangens  195 

Therefore,  a  transmission  of  the  hereditary  characters 
from  the  nucleus  to  the  cytoplasm1  must  in  some  way 
take  place  here,  and  the  observations  communicated  in 
the  previous  Section  furnish  important  arguments  for 
the  correctness  of  this  deduction. 

These  are  the  conclusions  that,  to  my  mind,  are  fully 
justified  by  the  facts  at  hand.  The  assumption  of  pan- 
gens  is  a  hypothesis  that  seems  to  me  indispensable  at  our 
present  state  of  knowledge.  To  my  mind  it  is  absolutely 
necessary  for  the  explanation  of  the  allied  relations  of 
organisms,  provided  that  this  explanation  is  attempted 
on  a  material  basis. 

I  shall  leave  now  these  general  considerations,  and 
attempt  to  describe  how  I  picture  to  myself  the  relation 
of  the  pangens  to  the  phenomena  of  cell-life.  I  am  per- 
fectly aware  of  the  fact  that  the  working  out  of  a 
hypothesis  to  its  extreme  consequences  leads  only  too 
easily  to  erroneous  conclusions,  and  is  of  value  for  science 
only  when  leading  to  definite  problems  that  can  be  solved 
experimentally.  I  shall  therefore  limit  myself  to  only 
one  hypothesis,  which,  it  seems  to  me,  recommends  it- 
self by  its  simplicity.  This  hypothesis,  with  the  deduc- 
tions resulting  directly  from  it,  will  form  the  subject 
of  this  last  section. 

The  hypothesis  reads  as  follows :  All  living  proto- 
plasm consists  of  pangens;  they  form  the  only  living 
elements  in  it. 

§  2.     All  Protoplasm  Composed  of  Pangens 

From  Hertwig's  renowned  discovery,  some  investi- 
gators have  inferred  that  only  the  nucleus  is  the  bearer 
of  hereditary  characters ;  that  they  are  entirely  restricted 

!By  cytoplasm  I  mean  all  the  protoplasm  except  the  nucleus. 


196         Pang  ens  in  the  Nucleus  and  Cytoplasm 

to  it.  To  my  mind  this  is  a  much  too  far-reaching  de- 
duction, and  without  justification.  The  fusion  of  the 
nuclei  during  fertilization  is  evidence  only  that  all  the 
hereditary  characters  must  be  represented  in  the  nucleus, 
but  this  fact  does  not  decide  that  they  cannot  be  present, 
in  addition,  in  the  cytoplasm. 

The  organs  of  the  fertilized  egg-cell  are  still  the  same 
as  those  of  the  unfertilized ;  the  young  plant  has  inherited 
from  the  mother  its  chromatophores  and  vacuoles  as  such. 
In  the  long  succession  of  cell-divisions  which  are  started 
by  the  fertilized  egg-cell,  those  organs,  multiplying 
steadily  by  division,  are  transmitted  each  time  to  the 
daughter-cells.  They  have,  so  to  speak,  their  independ- 
ent pedigree  in  addition  to  that  of  the  nucleus.  There 
is,  therefore,  an  additional  heredity  outside  the  nucleus. 

The  smallest  morphological  particles,  out  of  which 
the  chromatophores  are  built  up,  must  evidently  possess 
the  power  of  multiplying  independently,  otherwise  neither 
the  growth  nor  the  repeated  divisions  of  these  structures 
could  be  explained.  In  this  respect  these  particles  are 
obviously  similar  to  the  pangens  of  the  nucleus.  The 
power  of  producing  chlorophyll  must  be  present  in  a 
latent  state  in  certain  pangens  of  the  nucleus;  it  is  also 
inactive  in  the  smallest  particles  of  the  chromatophores, 
in  the  higher  plants,  as  long  as  the  respective  members 
are  in  darkness,  and  becomes  active  only  on  exposure  to 
light. 

We  shall  therefore  either  have  to  assume  chlorophyll- 
pangens  in  the  nucleus,  and  special  chlorophyll-forming 
particles  in  the  chromatophores,  or  identify  the  two,  and 
imagine  that  those  hypothetical  units  are  inactive  in  the 
nucleus,  and  become  active  only  when  they  pass  on  to 
the  chromatophores.  The  second  assumption  is  obviously 


All  Protoplasm  Composed  of  Pangens  197 

the  simpler  one ;  for  the  first  requires,  for  every  function, 
two  kinds  of  units,  which  multiply  by  growth  and  divi- 
sion, and  which  must  stand  in  such  mutual  relationship 
that  the  units  in  the  chromatophore  can  function  only 
in  the  manner  prescribed  by  the  respective  pangens  in 
the  nucleus. 

Precisely  the  same  argument  can  also  be  used  for  the 
other  characters  of  the  chromatophores,  and  for  the  other 
organs  of  the  protoplasts,  in  a  word,  for  all  hereditary 
characters. 

Let  us  consider  the  question  from  the  standpoint  of 
the  theory  of  descent.  In  the  first,  as  yet  non-nucleated 
organisms,  we  must  also,  as  a  matter  of  course,  regard 
the  individual  characters  as  being  connected  with  pangens. 
But  here  the  latter  must  evidently  lie  in  the  protoplasm. 
And.  as  soon  as  differentiation  advanced  so  far  that  not  all 
qualities  had  to  be  active  at  the  same  time,  active  and 
latent  pangens  must  in  these  simple  protoplasts,  have 
lain  side  by  side  and  intermingled.  According  to  age  and 
external  circumstances,  at  one  time  some,  at  another 
time  other  pangens  would  enter  into  activity.  Here  it 
would  be  quite  superfluous  to  assume,  for  each  function, 
two  kinds  of  units,  on  the  one  hand  latent  pangens, 
merely  having  charge  of  heredity,  and  on  the  other 
hand,  particles  which  might  express  the  latent  characters. 
The  assumption  that  the  same  pangens  can  be  either  ac- 
tive or  latent  according  to  circumstances,  is  evidently 
much  simpler  for  these  lower  organisms. 

It  can  hardly  be  doubted  that  protoplasm  consists  of 
most  minute  particles  which  are  able  to  multiply  independ- 
ently. This  is  indeed  the  real  attribute  of  life.  And  it 
also  seems  to  me  clear  that  we  should  regard  only  these 
particles  as  life-units,  and  everything  else,  such  as  pro- 


198         Pangens  in  the  Nucleus  and  Cytoplasm 

tein,  glucose,  and  salts,  present  only  in  the  water  of  im- 
bibition, as  secondary  to  them.  How  these  particles  are 
constituted,  whether  they  themselves  contain  water  of 
imbibition,  or  not,  and  how  the  visible  characters  are 
conditioned  by  their  structure,  we  do  not  know;  much 
less  are  we  acquainted  with  their  manner  of  dividing  and 
multiplying.  Apart  from  these  difficulties,  which  adhere 
to  any  theory,  the  assumption  that  these  particles  are 
identical  with  the  bearers  of  the  hereditary  traits,  is  ob- 
viously the  simplest  one  that  can  be  made  with  regard 
to  the  structure  of  living  matter. 

From  this  point  of  view,  the  origination  of  the  nucleus 
in  the  phylogenetic  differentiation  of  the  lowest  organ- 
isms, appears  to  us  as  an  extremely  practical  division  of 
labor.  Hitherto,  the  active  and  the  inactive  pangens  were 
lying  everywhere  in  the  protoplasm,  side  by  side  and 
intermingled.  And  the  higher  the  differentiation  that  had 
been  reached,  the  greater  would  be  the  number  of  diverse 
pangens,  in  the  same  protoplast;  and  the  greater,  also, 
would  have  to  be  the  number  of  the  latent  among  the 
active  ones.  The  latter  would  thereby  be  distributed  over 
a  relatively  large  space,  and  the  efficiency  of  the  whole 
must  therefore  suffer.  By  the  formation  of  the  nucleus 
this  situation  could  be  changed.  In  the  latter  the  inactive 
pangens  would  be  accumulated  and  stored;  the  active 
ones  could  come  nearer  each  other. 

Let  us  further  elaborate  the  picture.  As  soon  as  the 
moment  arrived  for  certain  pangens,  which  until  then 
had  been  inactive,  to  be  set  into  activity,  they  would  ob- 
viously pass  from  the  nucleus  into  the  cytoplasm.  But 
in  so  doing  they  would  retain  their  characters,  and  es- 
pecially their  power  to  grow  and  multiply.  Only  a  few 
like  pangens  would  therefore  have  to  leave  the  nucleus 


Active  and  Inactive  Pang  ens  199 

every  time  in  order,  by  further  multiplication,  to  impress 
the  characters  of  which  they  are  the  bearers,  on  a  given 
part  of  the  cytoplasm.  This  process  would  repeat  itself 
at  every  change  of  function  of  a  protoplast;  every  time 
new  pangens  would  leave  the  nucleus  in  order  to  become 
active.  In  this  way  the  whole  cytoplasm  would  soon 
consist  of  pangens  drawn  from  the  nucleus,  and  of  their 
descendants. 

§  j.    Active  and  Inactive  Pangens 

Darwin  has  already  emphasized  the  fact  that  the 
transmission  of  a  character  and  its  development,  even 
though  they  frequently  occur  conjointly,  are  yet  distinct 
powers.2  This  point,  derived  from  the  phenomena  of 
atavism,  has  attained  great  significance  in  cell-theory 
through  the  discovery  of  the  function  of  the  cell-nucleus. 
i  The  function  of  the  nucleus  is  transmission,  that  of  the 
cytoplasm,  development. 

Former  theories  assumed  a  complete  contrast  be- 
tween nucleus  and  cytoplasm,  imagining  hereditary  char- 
acters to  be  limited  to  the  former,  and  seeing  in  the  rest 
of  the  protoplasm  only  a  passive  substratum,  by  means 
of  which  the  nuclei  do  their  work.  Thus  the  nucleus 
became  the  essential  part  of  the  cell ;  not  only  did  it  dom- 
inate, but  also  completely  determine  the  functions.  But 
the  experiments  of  Nussbaum,  Gruber,  Klebs,  and  others 
have  taught  that  non-nucleated  fractional  parts  of  lower 
organisms  are  also  able  to  exercise  certain  functions. 
Especially  do  they  seem  to  possess  the  power  of  contin- 
uing later  those  functions  in  which  they  were  already 
engaged  before  being  detached.  Hence,  the  influence 

2Darwin,  The  Variation  of  Animals  and  Plants.  2:  381.  New 
York.  1900. 


200         Pang  ens  in  the  Nucleus  and  Cytoplasm 

of  the  nucleus,  for  such  functions  at  least,  need  not  be 
continuous;  if  the  functions  have  once  been  exercised 
they  can  continue  later  without  the  cooperation  of  the 
nucleus.* 

The  simplest  explanation  of  this  lies  obviously  in  our 
assumption  that  nucleus  and  cytoplasm  are  both  built  up 
from  the  same  pangens,  with  this  difference,  only,  that 
in  the  nucleus  every  kind  of  pangen  of  the  given  species 
is  represented,  while  in  the  remainder  of  the  protoplasm 
of  each  cell  essentially  only  those  are  present  which  shall 
attain  their  power  of  activity  in  it.  In  the  nucleus  most 
of  them  are  inactive,  that  is,  they  only  multiply.  Nat- 
urally there  must  be  also  some  active  pangens  in  the  nu- 
cleus, as,  for  example,  those  that  carry  out  the  intricate 
process  of  nuclear  division ;  but  this  does  not  affect  the 
main  point.  In  the  organs  of  the  protoplast  the  pangens 
can  continue  their  multiplication,  and,  to  all  appearances, 
they  probably  always  begin  here  with  a  relatively  great 
increase  in  number.  With  that  they  can  here  remain 
active  or  inactive  for  a  shorter  or  longer  period ;  or  they 
may  be  active  and  inactive  by  turns.  Some  become  active 
at  their  arrival,  others  later,  some  independently  from 
external  conditions,  others  again  only  as  a  reaction  to 
definite  stimuli  that  start  their  activity. 

The  most  remarkable  processes  that  take  place  in  the 
interior  of  the  nucleus  during  nuclear  division  are  quite 
in  harmony  with  the  assumption  of  pangens.  Most  in- 
vestigators regard  the  chromatic  thread  as  the  morpho- 

*Godlewski's  experiment,  in  which  non-nucleated  portions  of  sea- 
urchin's  eggs  were  fertilized  by  the  spermatozoa  of  a  crinoid,  is  now 
well  known.  The  resulting  larvae  manifested  only  maternal  charac- 
ters. In  the  fifth  edition  of  his  "Allgemcine  Physiologic,"  Jena,  1909, 
Verworn  cites  this  experiment  as  establishing  beyond  doubt  the  fact 
that  hereditary  substance  is  not  entirely  confined  to  the  nucleus.  Tr. 


The  Transportation  of  Pangens  201 

logical  place  where  the  material  bearers  of  the  hereditary 
qualities  are  stored.*  This  thread  would,  therefore,  con- 
sist of  pangens  united  into  smaller  and  larger  groups, 
and  it  shows,  in  its  thickest  portions  a  distinct  structure 
of  special  particles  strung  together.  We  can  entirely 
agree  with  the  opinion  of  Roux,  where  he  sees,  in  the 
longitudinal  splitting  of  the  nuclear  skein,  the  visible 
part  of  the  separation  of  the  maternal  factors  into  the  two 
halves  destined  for  the  two  daughter  cells.3  This  concep- 
tion is  in  most  complete  harmony  with  pangenesis. 

§  4.   The  Transportation  of  Pangens 

Our  hypothesis  that  all  protoplasm  consists  of  pan- 
gens, led  us  to  the  conclusion  that  all  kinds  of  pangens 
are  represented  in  the  nucleus.  Here,  most  of  them  are 
inactive,  while  in  the  remainder  of  the  protoplasm,  they 
can  become  active.  From  this  it  follows  that,  from  time 
to  time,  pangens  are  transported  from  the  nucleus  to  the 
other  organs  of  the  protoplast. 

I  am  quite  aware  that,  with  most  readers,  this  de- 
duction will  prove  the  chief  difficulty  against  my  view. 
The  pangens  are  invisible,  therefore  their  transportation 
eludes  observation.  It  is  true  that  the  experiments  of 
Nussbaum,  Gruber,  and  Klebs,  discussed  in  the  preceding 
Sections,  prove  that,  on  cutting  off  the  opportunity  of 
transportation,  the  functions  of  the  protoplast  are  very 
greatly  restricted,  but  there  is  here  a  possibility  of  many 
other  influences  being  at  work.  Therefore  I  should  here 
like  to  emphasize  the  fact  that,  by  rejecting  my  hypothe- 

*Cf.  the  Translator's  Preface,  p.  viii. 

3Roux.  Ueber  die  Bedeutung  der  Kerntheilnngsfiguren.  Leipzig. 
1883. 


202         Pangens  in  the  Nucleus  and  Cytoplasm 

sis,  one  does  not  arrive  at  a  satisfactory  view  of  the  re- 
lation between  nucleus  and  cytoplasm. 

If  my  hypothesis  is  rejected  and  the  prevailing  con- 
ception concerning  the  contrast  between  nucleus  and  cyto- 
plasm is  followed,  we  can  imagine  the  effect  of  the 
nucleus  to  be  either  dynamic  or  enzymatic. 

Strasburger  represents  the  first  view.  According  to 
him,  the  reciprocal  action  between  the  nucleus  and  the 
cytoplasm  is  a  dynamic  one,  meaning  that  it  takes  place 
without  transmission  of  substance.5  For  this  investigator 
has  never  been  able  to  discover,  in  his  extensive  studies, 
a  transmission  of  visible  particles.  "From  the  nucleus, 
molecular  excitations  are  transmitted  to  the  surrounding 
cytoplasm  which  dominate,  on  the  one  hand,  the  processes 
of  metabolism  in  the  cell,  and  on  the  other  hand,  give  a 
definite  character,  peculiar  to  the  species,  to  the  growth 
of  the  cytoplasm,  which  depends  on  nutrition."  As  long 
as  it  is  a  question  of  general  insight  only,  this  assumption 
is  sufficient,  but  as  soon  as  attention  is  directed  to  indi- 
vidual processes,  we  meet  with  insurmountable  difficulties. 
Morphological  phenomena  are  indeed  far  from  having 
been  sufficiently  analyzed  to  allow  a  true  understanding, 
but  in  the  meantime  we  can  turn  to  the  much  simpler 
chemical  processes. 

Let  us  select  an  example.  It  is  an  hereditary  charac- 
ter of  by  far  the  greatest  number  of  plants  to  produce 
malic  acid  for  the  purpose  of  preserving  their  turgor,  and 
to  store  it  in  their  cell-sap,  most  frequently  in  connection 
with  inorganic  bases.  We  cannot  imagine  the  secretion 

5Strasburger,  E.  Neue  Untersuchungen  uber  den  Befruchtungs- 
vorgang  bei  den  Phanerogamen,  p.  111.  1884.  See  also  Weismann, 
A.,  Die  Kontinuitdt  des  Keimplasmas  als  Grundlage  einer  Theorie 
der  Vererbung,  p.  28.  1885.  Cf.  Translator's  Preface,  p.  viii. 


The  Transportation  of  Pangens  203 

of  this  acid  otherwise,  than  by  means  of  definite  particles, 
which  have  this  power,  owing  to  their  molecular  consti- 
tution, and  which  might  best  be  likened  to  enzymes. 

There  is  no  difficulty  in  assuming  that  these  particles 
become  active  only  when  they  are  made  so  by  molecular 
excitations  from  the  nucleus,  and  I  do  not  doubt  that  such 
co-relations  frequently  occur.  But  the  difficulty  lies  in 
the  question  as  to  whence  the  cytoplasm  gets  these  par- 
ticles. Because,  obviously,  the  power  of  forming  malic 
acid  cannot  be  communicated  by  those  excitations  to  any 
kind  of  substratum.  Such  excitations  can  only  set  free 
a  function,  and  only  that  can  be  set  free  which  is  already 
present  potentially.  Whence  then  originate  the  malic  acid 
formers  of  the  cytoplasm? 

This  question  is  not  answered  by  the  dynamic  theory. 
But,  as  previously  stated,  hybrids  teach  us  that  similar  pa- 
ternal characters  can  be  inherited  from  the  father,  and 
therefore  be  transmitted  in  a  latent  state  in  the  sperm-nu- 
cleus. Hence  the  producers  of  the  malic  acid  must,  them- 
selves, be  derived  from  the  nuclei.  They  are  simply  the 
active  states  of  the  malic  acid  pangens  that  are  inactive  in 
the  nucleus.  And  the  same  must  evidently  hold,  in  a 
similar  manner,  of  all  the  other  hereditary  factors. 

In  this  way,  we  arrive  at  the  assumption  previously 
made,  that  the  pangens  of  the  cytoplasm  originate  from 
the  nuclei. 

Haberlandt  has  pointed  out  the  possibility  of  an  en- 
zymatic influence  of  the  nucleus  on  the  cytoplasm.  The 
significance  of  peculiar  positions  of  the  nucleus,  observed 
by  this  investigator,  in  the  vicinity  of  the  place  of  most 
vigorous  cell-activity,  remains,  according  to  him,  the  same, 
"if  that  influence  should  be  not  a  dynamic,  but  a  material 
one,  and  if,  consequently,  a  diffusion  of  certain  chemical 


204        Pangcns  in  the  Nucleus  and  Cytoplasm 

compounds,  secreted  by  the  nucleus,  should  take  place 
through  the  plasm  to  the  place  of  growth.  The  effective- 
ness of  these  substances  would  doubtless  be  dependent  on 
the  degree  of  cencentration  of  their  solution,  and  this  in 
such  a  way  that  the  cytoplasm  would  react  to  them  only 
at  a  certain  concentration."6 

But  in  order  to  react  in  a  definite  manner  on  the  sub- 
stance secreted  by  the  nucleus,  the  cytoplasm  must  already 
possess  the  requisite  characters.  Starch  will  react  to  a 
secretion  of  diastase,  but  not  all  kinds  of  substratum  will 
do  so.  Thus  the  assumption  of  enzymatic  effects  demands 
the  presence,  in  the  cytoplasm,  of  hereditary  characters, 
which  have  been  taken  from  the  nucleus. 

Therefore,  no  matter  how  strange  the  assumption  of 
a  transmission  of  pangens  from  the  nucleus  to  the  cyto- 
plasm may  appear  at  first  glance,  we  arrive  by  the  most 
various  ways  of  reasoning  at  a  recognition  of  its  correct- 
ness. 

An  important  question  is  that  of  the  time  when  this 
transportation  chiefly  occurs.  A  comparative  considera- 
tion of  the  various  forms  of  variability  will  in  the  end, 
it  is  hoped,  furnish  the  necessary  material  for  its  answer ; 
in  the  mean  time  we  may  assume  it  as  probable  that  im- 
mediately after  fertilization,  as  well  as  during  or  after 
every  cell-division,  such  a  transportation  takes  place.  Hy- 
brids, and  those  variations  that  affect  in  a  similar  man- 
ner all  the  members  of  a  plant,  argue  in  favor  of  the  first 
point,  and  for  the  other,  the  previously  discussed  phenom- 
ena of  dichogeny,  where  during  the  earliest  youth  of  an 
organ  its  later  nature  can  be  determined  by  external  in- 
fluences. When,  for  instance,  the  terminal  bud  of  a 
rhizome  grows  prematurely  and  turns  into  an  upward 

6Haberlandt,  G.  Ueber  die  Beziehungen  zwischen  Function  und 
Lage  des  Zellkcrncs,  p.  14,  note.  1887. 


The  Transportation  of  Pangens  205 

shoot,  or  the  primordium  of  a  transformed  leaf  becomes 
a  normal  leaf,  we  may  assume  that  other  pangens  have 
been  given  up  by  the  nucleus,  than  would  have  been  the 
case  without  artificial  interference.  Therefore,  in  that 
youthful  state,  the  normal  delivery  cannot  yet  have  come 
to  an  end.  When  grown  cells  are  stimulated  to  form 
callus  or  wound-cork  or,  as  in  Begonia,  to  produce  de 
novo  entire  plantlets,  it  is  to  be  supposed  that  the  pangens 
that  thereby  become  active  must  first  be  aroused  from 
their  latent  state. 

The  transportation  of  pangens,  and  their  conveyance 
to  the  proper  places,  demands  quite  special  arrangements, 
the  existence  of  which  many  a  reader  will  hardly  venture 
to  suspect.  But  who  would  have  dared,  ten  years  ago,  to 
assume  the  remarkably  complicated  structure  of  the  nu- 
cleus ?  We  must  be  as  sparing  as  possible  with  our  hypoth- 
eses, but  on  the  other  hand  we  must  not  be  blind  to  the 
fact  that  since  Mohl's  time,  the  investigation  of  the 
structure  of  the  protoplast  has  disclosed  more  and  more 
differentiations,  and  that,  most  likely,  we  are  still  far 
from  the  end. 

To  my  mind  the  currents  in  the  protoplasm  form  one 
arrangement  for  the  purpose  of  this  transmission.  Every- 
body knows  how  they  take  place  in  youthful  cells  at  paths 
that  radiate  from  the  nucleus,  and  more  recent  investiga- 
tions have  taught  how  they  frequently  connect  the  places 
of  greatest  activity  directly  with  the  nucleus. 

A  few  years  ago  the  conviction  that  these  little  cur- 
rents are  a  quite  common  peculiarity  of  plant-cells,  was 
far  from  being  prevalent.  The  phenomenon  was  imagined 
to  be  limited  to  a  number  of  instances.  Hanstein  has 
already  pointed  out  how  little  this  view  was  justified,7  and 
Velten  has  proven  the  presence  of  currents  in  all  plants 

7Hanstein,  Das  Protoplasma,  p.  155.    1880. 


206         Pangens  in  the  Nucleus  and  Cytoplasm 

examined  with  this  point  in  view.8  In  the  Botanische 
Zeitung  for  1885,  I  have  furnished  proof  that  mechanical 
contrivances  are  not  sufficient  for  the  transmission  of  the 
assimilated  nutrient  matter  in  plants,  and  that,  of  the 
processes  known  up  to  date,  it  can  only  be  accomplished 
by  the  currents  of  the  protoplasm.9 

In  this  connection  I  have  carefully  verified  Velten's 
statement,  and  have  confirmed  the  quite  common  exist- 
ence of  currents  in  vigorously  living  plants.10 

The  mechanical  possibility  of  a  transmission  of  pan- 
gens  is,  therefore,  sufficiently  assured  for  all  plant-cells. 
Only  one  difficulty  has  yet  to  be  overcome.  Following 
the  precedence  of  Hofmeister,  it  was  generally  assumed 
that  the  currents  in  the  cells  begin  only  at  the  end  of  the 
meristematic  period,  and  that,  until  that  time,  the  granu- 
lar plasm  is  in  a  state  of  rest.  Now  the  meristematic 
period  is  not  only  that  in  which  the  cells  originate,  but 
also  that  in  which  their  later  character  is  chiefly  deter- 
mined. Hence  it  is  in  this  very  period  that  we  must  place 
the  most  important  part  of  the  transportation  of  the 
pangens. 

But  Hofmeisters  statement  was  based  on  insufficient 
observations.  A  subsequent  investigation  by  Went,  with 
the  more  modern  methods,  led  to  a  quite  different  result.11 
The  movements  are  indeed  slow,  and  one  examination 
will  often  not  disclose  them.  But  if  the  observation  of 

8Velten,  W.  Ueber  die  Verbreitung  der  Protoplasmabewegungen 
im  Pflanzenreiche.  Bot.  Zeit.  30:  645.  1872. 

9Vries,  H.  de.  Ueber  die  Bedeutung  der  Circulation  und  der 
Rotation  des  Protoplasma  fur  den  Stofftransport  in  der  Pflanze.  Bo. 
Zeit,  43:  1.  1885. 

10Over  het  algemeen  voorkomen  van  circulatie  en  rotatie  in 
de  weepelcellen  der  planten,  Maandbl.  v.  Natuurw.  No.  6.  1884. 
Cf.  ibid.  No.  4,  1886,  and  Bot.  Zeit.  43:  1,  17.  1885. 

lxWent,  F.  A.  F.  C.  Die  Vermehrung  der  Normalen  Vacuolen 
durch  Theilung.  Jahrb.  Wiss.  Bot.  19:  329.  1888. 


Comparison  with  Darwin's  Hypothesis          207 

the  same  object  is  continued  for  hours  under  favorable 
life-conditions,  there  will  be  noticed  all  kinds  of  displace- 
ments, which  put  the  presence  of  slow  currents  beyond 
a  doubt. 

From  this  side,  therefore,  no  difficulty  stands  in  the 
way  of  the  assumption  that  the  transmission  of  the  pan- 
gens  in  plant-cells  is  accomplished  by  the  currents  of  the 
granular  plasm.  In  the  domain  of  animal  physiology  we 
are  far  from  possessing  the  necessary  knowledge  of  the 
currents  of  the  protoplasm.  But  then  the  difficulties  of 
investigating  are  here  considerably  greater  than  in  the 
plant-world. 

§  5.  Comparison  with  Darwin's  Transportation- 
Hypothesis 

Possibly  to  some  readers  there  will  appear  to  be  a 
great  similarity  between  the  assumption  of  a  transmission 
of  pangens  from  the  nucleus  to  the  other  organs  of  the 
protoplast,  as  described  in  the  previous  paragraphs,  and 
Darwin's  hypothesis  of  the  transportation  of  gemmules. 
However,  this  agreement  is  only  apparent  and  not  real. 
The  two  hypotheses  are  fundamentally  different  through- 
out. 

Darwin  assumed  a  transportation  of  gemmules 
through  the  entire  body ;  my  view  requires  only  a  move- 
ment within  the  narrow  limits  of  an  individual  cell.  But 
this  is  not  the  chief  difference.  In  the  gemmule-theory, 
the  particles  that  are  separated  from  a  cell  or  a  member 
can  again  enter  new  cells,  especially  the  germ-cells,  and 
thus  endow  them  with  new  hereditary  factors.  Not  only 
can  the  latter  then  reach  their  development  in  the  given 
germ-cell,  but  they  can  also  be  transmitted  to  all  its  de- 


208         Pangens  in  the  Nucleus  and  Cytoplasm 

scendents.  To  this  end,  however,  they  must,  according 
to  the  present  state  of  cell-anatomy  and  of  the  study  of 
fertilization,  be  received  into  the  nuclei.  The  hypothesis 
of  intracellular  pangenesis  obviously  does  not  make  such 
an  assumption;  the  pangens  that  have  once  left  the  nu- 
cleus do  not  have  to  return  to  it,  neither  into  the  nucleus 
of  the  same  cell,  nor  into  that  of  any  other. 

It  is  true  that,  with  our  present  anatomical  knowledge, 
the  possibility  of  a  transmission  of  pangens  from  one  cell 
to  another  cannot  be  denied.  The  researches  of  Tangl, 
Russow,  and  many  other  investigators  on  the  direct  con- 
nections of  the  protoplasts  of  neighboring  cells  by  means 
of  the  delicate  pore  canals  of  the  pits,  even  indicate  the 
path  on  which  such  a  passage  might  eventually  take  place. 
In  the  latex  vessels  the  currents  of  protoplasm  are  un- 
doubtedly not  limited  to  the  individual  constituent  cells, 
the  current  continuing  without  regard  to  the  former  cell- 
limits.  This  is  especially  the  case  with  the  mass-move- 
ment after  injuries,  and  probably  also  with  the  proper 
movements  of  the  granular  plasm  in  the  normal  state.  If 
we  assume  that  all  living  protoplasm  consists  of  pangens, 
their  passage  from  one  cell  to  another  cannot  be  denied 
here.  But  this  phenomenon  is  obviously  of  no  importance 
for  the  theory  of  heredity.  Similar  considerations  could 
be  made  for  other  cases  of  cell-fusions,  or  symplasts. 

The  mode  of  origin  of  the  secondary  pores  of  the 
Florideae,  discovered  by  Kolderup-Rosenvinge,12  is  also 
worthy  of  note.  The  cortical  cells,  e.  g.,  of  Polysiphonia, 
divide  in  the  usual  manner  with  preceding  nuclear  di- 
vision. But  one  part  contains  almost  the  entire  proto- 
plast and  the  other  but  a  small  corner  at  its  base.  The 

12Kolderup-Rosenvinge,  L.  Sur  la  formation  des  pores  second-- 
aires  chez  les  Polysiphonia.  Botanisk  Tidsskrift.  17:  10.  1888. 


Comparison  with  Darwin's  Hypothesis         209 

wall  arising  between  the  two  halves  forms  a  primary  pit. 
At  that  place  the  wall  between  the  separated  corner  and 
the  underlying  cell  is  dissolved,  and  contact  being  thus 
established  between  the  two  protoplasts,  they  fuse.  The 
old  poreless  cross-wall  is  thus  replaced  by  a  new  one  that 
contains  a  pore.  But  the  interesting  point  for  our  pur- 
pose is  the  circumstance  that  the  underlying  cell  has  now 
received  a  nucleus  from  its  upper  neighbor.  It  has  two 
nuclei,  and  later  it  becomes  multi-nuclear  by  nuclear 
divisions.  For  all  those  who  regard  the  nucleus  as  the 
bearer  of  the  herditary  endowment,  a  transmission  of  the 
latter  here  takes  place  from  one  cell  to  another.  But 
obviously  again  without  any  significance  for  the  theory 
of  heredity. 

The  possibility  of  a  transmission  of  material  bearers 
of  hereditary  characters  from  one  cell  to  another  can 
therefore  not  be  denied.  Further  investigations  will, 
without  doubt,  bring  to  light  other  facts  that  can  be  util- 
ized for  the  same  purpose.  And  that  here  and  there,  in 
plants,  processes  take  place  in  a  similar  way,  which  stand 
in  direct  relation  to  heredity  can,  of  course,  not  be  denied 
a  priori. 

But  it  is  quite  another  question  whether  such  a  trans- 
mission occurs  commonly,  and  plays  an  important  role 
in  the  transmission  of  hereditary  characters  in  the  whole 
plant  and  animal  world. 

Anatomical  facts  alone  are  not  sufficient  to  answer  this 
question.  From  them,  only  the  possibility  of  a  transmis- 
sion can  be  deduced  or,  more  correctly  speaking,  the  con- 
clusion that  our  present  knowledge  does  not  furnish  any 
reasons  which  would  make  that  transmission  appear  im- 
possible. It  may  be  that  such  a  thing  will  be  discovered 
later.  But  it  is  not  likely  that  anybody  will  think  it  is 


210         Pang  ens  in  the  Nucleus  and  Cytoplasm 

therefore  permissible  to  infer  the  actual  occurrence  of  a 
general  intercellular  transmission  of  the  bearers  of  hered- 
itary properties. 

Hence,  the  answer  to  the  question  must  be  looked  for 
in  a  quite  different  field.  The  theory  of  heredity  must 
tell  us  whether  there  are  facts  for  the  explanation  of 
which  the  assumption  of  an  intercellular  transmission  is 
indispensable. 

To  my  mind,  this  is  not  the  case,  as  I  have  already 
stated  in  the  Introduction.  I  have  there  referred  to  Weis- 
mann's  writings,  which  contain  copious  demonstrations 
that  all  observations  which  so  far  seemed  to  demand  such 
an  assumption,  could  in  reality  have  been  explained  as 
well,  and  in  most  cases  better,  without  them. 

Especially  should  the  so-called  heredity  of  acquired 
characters  be  mentioned  here.  I  have  previously,  in  an- 
other place,  drawn  attention  to  the  fact  that  in  many  cases 
we  have  here  to  deal  with  malformations.13  If  we  limit 
the  meaning  of  that  expression  to  the  variations  which 
have  arisen  on  the  somatic  tracks,  and  ask  whether  these 
can  be  transmitted  to  the  germ-tracks  of  the  organism, 
then  the  question  has  a  clear  meaning.  In  that  case  we 
can  join  Weismann  in  quietly  answering,  no.  But,  if  we 
also  call  such  characters  as  may  have  originated  on  the 
germ-tracks  acquired,  the  question  is  no  longer  of  any 
significance  for  the  problem  which  occupies  us  here.14 

In  botany  graft-hybrids  and  xenia  are  mentioned  as 

13"Over  steriele  Mais-planten,"  Jaarboek  v.  h.  Vlaamsch  kruidk. 
Genootschap,  Bd.  1.  Gent.  1889. 

14The  conception  of  germ-tracks  and  somatic  tracks  in  the  sense 
developed  in  the  first  Section  of  this  second  Part  may  contribute 
much,  in  this  connection,  to  help  the  mutual  understanding.  See  also 
e.  g.,  in  regard  to  Eimer's  discussions,  his  work :  Die  Entstehung  der 
Art  en  auf  Grund  von  Vererben  erworbener  Eigenschaften.  Theil  1. 


Xenia  211 

arguments  for  an  intercellular  transmission  of  hereditary 
qualities.  But  both  groups  of  phenomena  are  much  in 
need  of  being  critically  investigated  before  they  can  be 
reliably  employed  in  this  way.  The  transmission  of  the 
hereditary  characters  of  the  crown-graft  to  its  stock15  has, 
to  my  mind,  never  been  scientifically  proven,  and  never 
will  be,  as  long  as  new  experiments  are  not  made,  in 
which  the  variations  of  the  stock  itself,  are  thoroughly 
studied  and  have  become  well  known.  Because,  until 
then,  the  possibility  is  not  excluded  that  this  variability 
of  the  stock  itself  forms  the  most  important  factor  in  the 
phenomena  that  have  been  observed. 

The  cases  where  the  pollen  is  supposed  to  have  trans- 
mitted hereditary  characters  outside  the  fertilized  egg- 
cell  and  the  embryo  issuing  from  it,  to  the  tissues  of  the 
maternal  fruit,  have  been  carefully  arranged  by  Focke 
under  the  name  xenia.16  And  his  review  shows 
plainly  that  here  one  has  to  deal  with  exceptional  cases 
which  have  never  yet  been  thoroughly  studied  and  suffi- 
ciently controlled.17  And  I  think  that,  without  a  control, 
based  on  critical  examination,  these  data  cannot  be  given 
that  far-reaching  significance  that  would  make  them  the 

15Cf.  the  critical  summary  of  the  material  for  observation  bear- 
ing on  this  point,  by  H.  Lindemuth,  Uber  Vegetative  Bastarderzeug- 
ung  durch  Impfung.  Landw.  Jahrb.  7:  887.  1878. 

16Focke,  Die  Pflanzenmischlinge,  pp.  510-518.  1881.  [See  also, 
Webber,  H.  J.  Xenia,  or  the  immediate  effect  of  pollen  on  Maize. 
U.  S.  Dept.  Agr.  Div.  Veg.  Physiol  Pathol  Bull.  22.  Sept.  12,  1900; 
Correns,  C.  Untersuchungen  iiber  die  Xenien  bei  Zea  Mays.  Ber. 
Deut.  Bot.  Ges.  17:  410.  1899.  TV.] 

17The  best  known  instance  of  Xenia,  that  of  corn,  has  since  been 
shown  to  be  of  a  different  nature,  consisting  in  the  hybridization  of 
the  endosperm  in  the  process  of  double  fertilization.  See  de  Vries, 
Sur  la  fecondation  hybride  de  1'albumen.  Compt.  Rendus  Acad.  Sci., 
Paris,  129:  973.  1899,  and  Sur  la  fecondation  hybride  de  1'  endo- 
sperme  chez  le  Mais.  Revue  generate  de  Botanique.  11:  129.  1900. 


212         Pang  ens  in  the  Nucleus  and  Cytoplasm 

bases  for  an  assumption  of  an  actual  intercellular  trans- 
mission of  hereditary  qualities. 

The  facts  of  heredity  so  far  known,  do  not,  to  my 
mind,  make  the  assumption  of  an  intercellular  transmis- 
sion of  pahgens  necessary.  When  the  pangens  have  once 
left  the  nucleus  they  do  not  need  the  power  of  penetrating 
back  into  that  nor  into  any  other  nucleus.  The  pedigree  of 
the  pangens  lies  in  the  nuclei,  and  its  protoplasmic  side- 
branchings  all  end  blindly,  although  often  only  after  many 
cell-divisions. 

I  believe  that  the  passage  of  the  pangens  from  the 
nuclei  is  a  necessary  conclusion  of  our  present  knowledge 
concerning  the  physiological  significance  of  the  nuclei. 
I  need  not  assume  a  penetration  of  the  extruded  pangens 
or  their  descendents  into  other  nuclei.  And  this  hypothe- 
sis would  be  inevitable  if  one  were  to  connect  Darwin's 
transportation  of  gemmules  with  the  results  of  more  re- 
cent cell-study.  In  this  case  one  would  have  to  resort  to 
a  new  ancillary  hypothesis  in  order  to  explain  facts, 
which,  according  to  the  discussions  mentioned  above,  do 
not  at  all  require  such  an  explanation. 

Let  us  summarize  the  difference  between  the  two 
transmission  hypotheses.  The  pangens  of  the  intracellu- 
lar  pangenesis,  having  once  left  the  nucleus,  need  never 
re-enter  it.  For  the  gemmules  of  Darwin's  transporta- 
tion hypothesis,  however,  this  power  is  the  essential  con- 
dition, because  without  it,  the  hereditary  properties  of 
which  they  are  the  bearers,  can  never  develop  into  visible 
characters  in  the  descendants  of  the  respective  germ-cells. 

§  6.     The  Multiplication  of  Pangens 

The  hypothesis,  that  the  entire  living  substance  of  a 
cell  is  built  up  of  pangens,  naturally  implies  that  in  every 


The  Multiplication  of  Pang  ens  213 

protoplast  every  kind  of  pangen  must  be  represented  in 
great  numbers.  In  addition,  the  relative  number  of  the 
bearers  of  the  individual  hereditary  characters  is  of  very 
great  importance.  In  the  cytoplasm  it  determines  the 
function  of  the  individual  organs,  in  the  nucleus  the  power 
of  inheritance.  If  a  new  character  in  the  nucleus  is  rep- 
resented by  only  a  few  like  pangens,  the  likelihood  of  this 
character  becoming  visible,  is  evidently  very  small.  But 
the  greater  the  number  of  those  pangens,  in  comparison 
with  the  others,  the  more  prominent  will  the  character 
appear.  From  seeds  of  a  twisted  specimen  of  Dipsacus 
sylvestris  I  have  grown  over  1 ,600  plants,  of  which  only 
two  showed  torsion  of  the  stem.  The  pangens  which 
caused  this  torsion  must,  therefore,  have  been  in  such 
relatively  small  numbers  that  their  chance  of  becoming 
active  amounted  to  1  per  1,000  at  the  most.  In  other 
young  varieties  this  proportion  is  more  favorable,  and, 
by  making  the  right  selection,  that  chance  increases  quite 
considerably  in  the  course  of  a  few  generations.  The 
simplest  explanation  for  this  is  obviously,  that  by  breed- 
ing those  specimens  in  which  the  characteristic  is  repre- 
sented by  the  greatest  number  of  like  pangens,  the  relative 
number  of  these  is  gradually  increased. 

I  have  repeatedly  emphasized  the  fact  that,  according 
to  my  hypothesis,  the  pangens  can  multiply  in  the  nu- 
cleus as  well  as  in  the  cytoplasm.  This  multiplication  is 
of  the  same  order  as  that  of  the  cells  and  of  the  organ- 
isms themselves.  When  a  large  tree  bears,  every  year, 
thousands  of  seeds,  the  pangens  of  the  egg-cell  from 
which  the  tree  has  grown,  must  have  multiplied  in  an  in- 
credible manner.  And  the  same  thing  is  taught  by  the 
enormous  number  of  eggs  that  a  single  tape-worm  can 
produce.  In  the  face  of  such  phenomena  the  multiplica- 


214  Two  Kinds  of  Variation 

tion  of  the  pangens  in  the  cytoplasm  of  an  individual  cell 
is  only  minimal. 

The  giving  off  of  the  pangens  by  the  nucleus  must,  as 
a  matter  of  course,  always  be  done  in  such  a  way  that  all 
kinds  of  pangens  remain  represented  in  the  nucleus.  Al- 
ways only  a  relatively  small  number  of  like  pangens  must 
leave  the  nucleus.  The  division  of  the  nuclei,  however, 
must  take  place  in  such  a  way  that  all  the  different  kinds 
of  pangens  are  evenly  distributed  over  the  two  daughter- 
cells.  Only  in  certain  somatarchic  cell-divisions18  is  there 
a  deviation  from  this  regularity. 

The  two  kinds  of  variability  which  Darwin  distin- 
guishes on  the  ground  of  pangenesis,  are  naturally  also  to 
be  deduced  from  the  description  here  given.19  Fluctuating 
variability  is  simply  based  on  the  varying  numerical  rela- 
tion of  the  individual  kinds  of  pangens,  which  relation 
can  indeed  be  changed  by  their  multiplication  and  under 
the  influence  of  external  circumstances,  but  most  quickly 
by  breeding  selection.  The  "species- forming"  variabil- 
ity,20 that  process  by  which  the  differentiation  of  living 
forms  has  come  about,  in  its  main  lines,  must  essentially 
be  reduced  to  the  fact  that  the  pangens,  in  their  division, 
produce,  as  a  rule,  two  new  pangens  that  are  like  the 
original  one,  but  that  exceptionally  these  two  new  pangens 
may  be  dissimilar.  Both  forms  will  then  multiply,  and 
the  new  one  will  tend  to  exercise  its  influence  on  the  visi- 
ble characters  of  the  organism. 

In  harmony  with  this  is  the  idea  that  we  must  imagine 
the  higher  organisms  to  be  composed  of  a  greater  number 
of  unlike  pangens  than  the  lower  ones. 

18Cf.  pp.  102  and  107. 

19Cf.  p.  74. 

20Now  commonly  called  mutability  (de  V.    1909). 


CHAPTER  II 
SUMMARY 

§  7.    Summary  of  the  Hypothesis  of  Intracellular 

Pangenesis 

The  view  of  Darwin  (apart  from  the  hypothesis  of 
the  transportation  of  gemmules  through  the  entire  body), 
that  the  individual  hereditary  qualities  are  dependent  on 
individual  material  bearers  in  the  living  substance  of 
cells,  I  call  pangenesis.  These  bearers  I  call  pangens. 
Every  hereditary  character,  no  matter  in  how  many  spe- 
cies it  may  be  found,  has  its  special  kind  of  pangen.  In 
every  organism  many  such  kinds  of  pangens  are  assem- 
bled, and,  the  higher  the  differentiation  that  has  been 
reached,  the  more  there  are. 

The  hypothesis  that  all  living  protoplasm  is  built 
up  of  pangens,  I  call  intracellular  pangenesis.  In  the 
nucleus  every  kind  of  pangen  of  the  given  individual 
is  represented;  the  remaining  protoplasm  in  every  cell 
contains  chiefly  only  those  that  are  to  become  active  in  it. 
This  hypothesis  leads  to  the  following  conclusions.  With 
the  exception  of  those  kinds  of  pangens  that  become  di- 
rectly active  in  the  nucleus,  as  for  example  those  that 
dominate  nuclear  division,  all  the  others  have  to  leave  the 
nucleus  in  order  to  become  active.  But  most  of  the  pan- 
gens of  every  sort  remain  in  the  nuclei,  where  they  multi- 
ply, partly  for  the  purpose  of  nuclear  division,  partly  in 
order  to  pass  on  to  the  protoplasm.  This  delivery  always 
involves  only  the  kinds  of  pangens  that  have  to  begin  to 


216          Summary  of  Intracellular  Pangenesis 

function.  During  this  passage  they  can  be  transported 
by  the  currents  of  the  protoplasm  and  carried  into  the 
various  organs  of  the  protoplasts.  Here  they  unite  with 
the  pan  gens  that  are  already  present,  multiply,  and 
begin  their  activity.  All  protoplasm  consists  of  such 
pangens,  derived  at  different  times  from  the  nucleus,  to- 
gether with  their  descendants.  There  is  in  it  no  other 
living  basis. 

The  elaboration  of  this  hypothesis,  given  in  the  pre- 
ceeding  chapters,  is  only  an  outline,  the  purpose  of  which 
was  to  make  the  main  idea  comprehensible.  It  is,  for  the 
present,  the  simplest  form  in  which  pangenesis  can  accom- 
modate itself  to  our  present  knowledge  of  the  structure 
of  the  cell.  In  details  I  am  well  aware  of  not  having  been 
able  always  to  find  the  right  explanation.  But  the  only 
object  I  had  in  mind  was  to  demonstrate  how  easily  the 
greatly  misjudged  pangenesis  covers  all  the  facts  discov- 
ered since  its  establishment ! 


FERTILIZATION  AND  HYBRIDIZATION 
A  Paper 

read  at  the  151st  annual  meeting  to  the  Dutch  Society  of 
Science  in  Haarlem,  May  16,  1903 


The  essay  on  "Fertilization  and  Hybridization"  was  read  in 
Haarlem  in  the  Dutch  language,  and  appears  here  in  an  enlarged 
form.  My  conception  of  the  life-processes  in  the  nuclei  is  chiefly 
based  on  the  renowned  investigations  of  van  Beneden  and  of  Boveri, 
as  well  as  the  most  recent  researches  by  Conklin  (Contr.  Zool.  Lab. 
Pennsylvania,  XII,  192),  Sutton  (Biol.  Bull.  IV,  Dec.,  1902),  Eisen, 
(Jour.  Morphol  XVII,  1),  Errera  (Revue  Scientif.  Feb.,  1903),  and 
of  many  others.  For  the  literature  I  refer  to  E.  B.  Wilson,  The 
Cell  in  Development  and  Inheritance,  and  V.  Hacker,  Praxis  und 
Theorie  der  Zellen-und  Befruchtungslehre. 

My  presentation  of  the  processes  of  fertilization  and  hybridiza- 
tion is  an  outcome  of  the  experiments  which  I  have  described  in  the 
second  volume  of  my  Mutationstheorie  (Leipsic,  Veit  &  Co.,  1901- 
1903.  English  translation  by  Open  Court  Publishing  Co.,  1909-1910.) 

H  de  V. 


FERTILIZATION  AND  HYBRIDIZATION 

"Vom  Vater  hab'  ich  die  Statur, 
Des  Lebens  ernstes  Ftihrcn, 
Vom  Miitterchen  die  Frohnatur 
Und  Lust  zu  fabuliren."1 

In  these  lines  lies  the  whole  problem  of  heredity  and 
fertilization.  What  everybody  can  see,  Goethe  has  voiced 
clearly  and  concisely  in  beautiful,  simple  words.  We  have 
one  part  from  the  father,  the  other  from  the  mother.  Or, 
as  it  is  now  usually  put,  the  hereditary  characters  of  the 
two  parents  are  combined  in  the  offspring. 

It  became  the  problem  of  scientific  investigation  to 
seek  out  the  cause  of  this  phenomenon.  It  could  not  be 
limited  to  man.  The  law  mentioned  by  Goethe1  must  be 
general,  it  must  be  true  of  the  entire  plant  and  animal 
world,  wherever  two  beings  unite  for  the  production  of 
progeny.  Furthermore  it  cannot  concern  ordinary  fertil- 
izations only,  but  also  those  abnormal  cases  in  which  unlike 
individuals,  belonging  to  different  varieties  or  species, 
fertilize  each  other.  The  products  of  such  crosses  we 
call  hybrids,  and  for  science  they  possess  the  great  im- 
portance that,  in  them,  the  manner  in  which  the  charac- 
tertistics  of  the  parents  are  combined  can  be  studied  more 
easily  and  clearly  than  in  the  children  of  a  normal  union. 
For  the  more  the  parents  differ  from  each  other,  with 
the  greater  certainty  must  it  be  possible  to  determine  the 
share  of  each  in  the  characteristics  of  the  offspring. 

1  Goethe,  "Spriiche  in  Reimen,"  Gesammelte  Werke,  III,  83,  1871. 


220  fertilization  and  Hybridization 

Everywhere  this  law  is  confirmed,  that  the  child  in- 
herits one  part  of  its  nature  from  the  father,  the  other 
from  the  mother.  The  child  is,  therefore,  on  the  whole, 
a  double  being,  with  twofold  qualities,  more  or  less  dis- 
tinctly separated,  that  may  still  be  traced  back  to  their  ori- 
gin. This  principle  of  duality,  as  we  might  call  it,  domi- 
nates the  entire  theory  of  heredity ;  it  forms  the  thread  that 
binds  together  apparently  separated  cases;  it  serves  as  a 
guidance  for  the  whole  investigation. 

This  investigation  occupies  two  different  fields.  On 
the  one  hand  we  have  experimental  research,  on  the  other 
hand  microscopical.  Physiology  ascertains  the  relations 
of  the  offspring  to  their  parents ;  it  analyzes  their  charac- 
teristics into  their  individual  units,  and  tries  to  demon- 
strate their  origin.  The  history  of  development  discloses 
to  us  the  corresponding  microscopic  processes;  it  looks 
for  the  smallest  visible  bearers  of  heredity  in  the  cell,  and 
investigates  how  they  are  maintained  during  life,  and  how, 
during  fertilization,  they  pass  on  from  father  and  mother 
to  the  offspring. 

Few  investigators  master  both  provinces ;  their  extent 
is  much  too  great  for  that.  And  especially  has  the  study 
of  hybrids  so  greatly  advanced  in  recent  years,  that  even 
here  a  division  of  labor  will  soon  be  necessary.  Both  lines 
of  work  have  therefore  developed  more  or  less  indepen- 
dently of  each  other.  In  both,  the  main  features  of  the 
problem  begin  gradually  to  arise  out  of  the  abundance  of 
individual  phenomena.  And  thereby  there  is  disclosed, 
one  might  almost  say,  beyond  all  expectation,  an  agree- 
ment in  the  results  of  both  lines  of  investigation,  which 
is  so  great,  that  almost  everywhere  the  physiological  pro- 
cesses are  reflected  in  the  microscopically  visible  changes. 

It  is  true  that  the  final  analysis  lies  yet  beyond  the 


The  Double  Nature  of  Organisms  221 

limits  of  our  present  microscopical  vision.  Compared 
with  the  enormous  complexity  of  the  herditary  characters 
of  the  organisms  the  anatomical  structure  of  the  cells  and 
their  nuclei,  as  it  is  known  to  us,  is  much  too  simple.  The 
individual  traits  of  father  and  mother  can  not  yet  be  found 
in  the  cells  of  the  offspring,  but  the  investigations  of  most 
recent  times  indicate  clearly  that  here  also  the  limits  of 
knowledge  are  being  constantly  extended. 

The  double  nature  of  all  beings  that  have  sprung  into 
existence  through  fertilization,  is  seen  in  their  external 
appearance,  as  well  as  in  the  finest  structure  of  their  nu- 
clei. The  principle  of  duality  obtains  everywhere,  even  if, 
in  individual  cases,  the  demonstration  of  it  is  yet  in  its 
beginnings.  But  as  far  as  the  visible  marks  can  be  an- 
alyzed and  the  individual  component  parts  of  the  nuclei 
can  be  traced,  so  far  can  the  validity  of  the  principle  be 
proven  even  at  present. 

Let  us  consider  first  the  external  part,  then  the  inter- 
nal. 

Goethe  derived  his  stature  from  his  father,  and  not 
from  his  mother,  and  it  was  not  a  stature  between  the 
two.  The  sum  total  of  his  qualities  he  had  partly  from 
his  father,  partly  from  his  mother.  The  illustration  ex- 
plains the  rule  in  a  clear  manner.  In  the  offspring  the 
characters  of  the  parents  are  combined.  Not  always  does 
the  child  get  an  even  half  from  each;  on  the  contrary,  as 
everybody  knows,  it  resembles  the  mother  more  in  some 
respects,  and  the  father  more  in  others. 

It  is  exactly  the  same  with  hybrids.  With  them  a 
single  character  is  generally  derived  either  from  the  father 
or  from  the  mother.  The  hybrids  of  white  and  blue  flow- 
ers usually  bloom  blue,  those  of  a  hairy  or  a  thorny 
parent  crossed  by  one  without  hairs  or  thorns  are  usually 


222  Fertilisation  and  Hybridisation 

hairy  or  thorny.  The  crossing  of  a  common  evening- 
primrose  with  a  large-flowered  species  results  in  a  flower 
of  the  size  of  the  former.  But,  if  there  are  two  or  more 
points  of  difference  they  may  be  transmitted  to  the  chil- 
dren partly  by  the  one  parent  and  partly  by  the  other,  and 
it  is  thereby  possible  in  practice  to  combine  the  good  char- 
acters of  two  varieties  into  a  single  race.  Thus  has  Rim- 
pau  created  a  series  of  hybrid-races  of  wheat,  and  Lemoine 
has  produced  his  large-blooming  sword-lilies,  able  to  with- 
stand the  winter,  and  thus  have  originated,  in  agriculture 
and  horticulture,  the  countless  hybrids,  in  which  the  fa- 
vorable characteristics  of  various  varieties  are  combined 
with  more  or  less  diversity.  Combined,  or  as  we  usually 
say,  mixed ;  though  this  is  an  expression  which  makes  us 
only  too  easily  lose  sight  of  the  independence  of  the  in- 
dividual factors  in  the  mixture. 

This  independence  is  frequently  difficult  to  demon- 
strate in  the  mixtures,  that  is,  in  the  characteristics  of  the 
hybrids.  Our  means  of  differentiation  only  too  frequently 
prove  insufficient.  In  the  clear  cases,  however,  it  appears 
very  distinctly,  and  the  greater  the  number  of  hybrids  that 
are  studied  accurately  and  thoroughly,  the  more  generally 
is  the  validity  of  the  principle  established. 

If,  for  example,  we  find  combined  in  a  wheat-hybrid, 
the  loose  ear  of  the  mother-plant,  with  the  lack  of  awns 
in  the  father,  the  share  of  each  appears  simple  and  clear. 
In  the  mixture  of  the  characteristics  these  two  are  so  far 
apart,  that  they  are  always  easily  recognized.  How  are 
such  characters  united  in  the  hybrid  ?  Are  they  fused  into 
one  whole,  or  do  they  simply  lie  loosely  side  by  side  ? 

The  splittings,  which  occur  regularly  in  many  hybrids, 
when  propagated  by  seed,  and  also,  in  the  case  of  a  few,  in 
vegetative  propagation,  give  us  an  answer  to  this  question. 


Cytisus  Adami  223 

Of  the  last  kind  the  Cytisus  Adami  serves  as  the  most 
beautiful  and  striking  instance.  It  is  a  hybrid  between 
C.  Laburnum  and  C.  purpureus.  Unfortunately  its  great 
significance  for  the  main  features  of  the  whole  problem 
has  been  underrated  for  a  long  time  owing  to  the  fable 
of  its  having  originated  as  a  graft.  As  a  matter  of  fact, 
no  hybrids  are  obtained  by  grafting,  no  matter  how  great 
the  mutual  influence  of  the  wild  stock  and  the  crown 
graft.  As  far  as  historical  evidence  goes,  the  Cytisus 
Adami  has  always  been  propagated  by  grafts  since  its  first 
appearance,  but  it  did  not  originally  spring  into  existence 
in  this  way.2 

This  tree  teaches  us  how  the  qualities  of  the  two  pa- 
rents are  combined.  Ordinarily  they  occur  mixed,  the 
leaves  as  well  as  the  flowers  having  some  features  of  the 
Laburnum  and  others  of  the  purpureus.  The  totality  of 
the  characters  lies,  therefore  midway  between  the  two  pa- 
rents. But  splittings  do  occur,  and  not  at  all  rarely,  or 
rather  so  commonly,  that  indeed  every  specimen  of  the 
hybrid,  if  not  too  small,  will  show  them.  In  these  split- 
tings the  types  of  father  and  mother  separate  sharply  and 
completely.  Some  twigs  will  grow  that  are  purely  La- 
burnum, while  others  are  only  purpureus.  The  former 
are  vigorous  and  long-lived,  the  latter  remain  weak  and 
often  die  after  a  few  years,  which  is  the  reason  for  their 
being  seen  less  frequently.  But  even  in  this  point  they 
resemble  exactly  the  respective  parents. 

Within  the  hybrid,  the  bearers  of  the  parental  charac- 
ters are  therefore  arranged  in  such  a  manner  that,  so  to 
speak,  they  can  be  completely  separated,  at  any  moment, 

2Strasburger  (Jahrb.  Wiss.  Bot.  42:  69-70.  1905.)  finds  entire 
absence  of  any  cytological  evidence  that  C.  Adami  originated  as  a 
graft-hybrid.  Tr. 


224  Fertilisation  and  Hybridisation 

by  a  simple  cut.  And,  if  not  by  a  simple  cut,  then  at  least 
by  a  physiological  splitting,  which  passes  exactly  between 
the  two  parental  groups  and  does  not  leave  in  one  of  them 
any  trace  of  the  other. 

In  this  manner  we  have  to  picture  to  ourselves,  in  a 
general  way,  the  internal,  invisible  structure  of  the  hy- 
brids. The  bearers  of  the  characters  of  both  parents  are 
intimately  connected,  and  together  dominate  the  visible 
characteristics.  But  they  are  not,  by  any  means,  fused 
into  a  new  indivisible  entity.  They  form  twins,  but  re- 
main separable  for  life. 

In  all  nature  there  is  probably  not  another  such  beauti- 
ful instance  of  splitting  as  the  above-mentioned  Cytisus. 
But  with  lesser  differences  between  the  parents,  splittings 
of  the  parental  types  occur  frequently  in  the  vegetative  life 
of  hybrids.  Many  horticultural  plants,  and  especially  the 
bulbous  plants,  furnish  instances  thereof;  peas,  corn, 
wood-sorrel,  anagallis,  oranges,  and  several  others  are 
known  instances.  The  fruits  that  are  half  lemon  and  half 
orange,  belong  doubtless  to  this  group.  Among  the  hy- 
brids of  the  common  and  the  thornless  thornapple  (Datura 
Stramonium'),  individuals  have  been  found,  although  very 
rarely,  that  showed  a  similar  splitting,  and  which  even 
bore  on  the  same  fruit  armed,  as  well  as  thornless  cells. 
In  my  garden,  I  cultivated,  for  many  years,  a  Veronica 
longifolia  which  was  a  hybrid  from  the  blue  species  and 
the  white  variety,  and  correspondingly  had  blue  flowers. 
But  from  time  to  time  splittings  occurred ;  either  one  single 
spike  bloomed  white,  or  a  few  isolated  white  flowers  ap- 
peared on  an  otherwise  blue  spike. 

During  the  entire  life,  up  to  the  time  of  the  formation 
of  the  reproductive  cells  this  internal  dualism  manifests 
itself  in  this  way.  Sometimes  proofs  of  it  are  even  found 


The  Double  Nature  of  Organisms  225 

in  the  anatomical  structure  of  the  tissues,  and  of  the  indi- 
vidual cells,  where  the  parental  characters  are  set  free  and 
a  mosaic-like  structure  results. 

MacFarlane,  who  has  made  the  most  thorough  study 
of  the  anatomical  structure  of  hybrids,  recognizes  every- 
where the  principle  of  duality,  and  goes  so  far  as  to  regard 
every  individual  vegetative  cell  of  a  hybrid  as  a  herma- 
phrodite formation.  And  the  renowned  French  investi- 
gator of  hybrids,  Naudin,  also  expressed  himself  about 
forty  years  ago  in  a  similar  manner.  "U hybrid*  est  une 
mosaique  vivante"  said  he;  we  do  not  recognize  the  in- 
dividual parts  as  long  as  they  remain  intimately  blended, 
but  occasionally  they  separate  and  then  we  are  able  to 
distinguish  them. 

We  therefore  regard  it  as  established  that,  in  the  chil- 
dren, the  inheritances  from  the  fathers  and  mothers  are 
indeed  combined,  but  not  fused  into  a  new  entity.  Acting 
always  conjointly  under  ordinary  circumstances,  they  yet 
do  not  lose  the  power  of  separating  occasionally. 

But  now  arises  the  question  as  to  what  is  anatomically 
visible  of  this  union.  Can  the  dualistic  formation  be  ob- 
served within  the  cell  ?  Do  the  parental  inheritances,  here 
too,  lie  side  by  side  as  twins  ? 

The  hereditary  characters  are  contained  in  the  nuclei, 
as  was  first  declared  by  Haeckel,  and  later  demonstrated 
by  O.  Hertwig,  and,  for  plants,  by  Strasburger.  This  im- 
portant law  forms,  for  the  present,  the  basis  of  the  whole 
anatomical  theory  of  heredity,  and  is  recognized  as  such 
by  all  investigators.  We  may,  therefore,  expect  to  find  in 
the  nuclei,  as  well,  the  dualism  of  the  parental  qualities. 

Every  cell,  as  a  rule,  possesses  a  nucleus.  This  nucleus 
dominates  the  life-activity,  and  although  the  current  func- 
tions can  run  their  course  without  it,  no  new  ones  can  be 


226  Fertilization  and  Hybridization 

introduced.  In  certain  filamentous  algae  (Spirogyra)  Ge- 
rassimow  succeeded  in  producing  cells  without  nuclei; 
they  retained  life  for  several  weeks,  feeding  vigorously, 
but  nevertheless  they  always  perished  without  any  repro- 
duction. In  some  tissue-cells  the  nucleus  is  constantly  in 
motion,  and  according  to  Haberlandt's  investigations,  it 
stops  longest  where  the  work  of  the  cell  is  most  pro- 
nounced for  the  time  being,  as  for  instance  in  unilateral 
growth,  the  formation  of  hair,  local  accumulation  of 
chlorophyll,  etc. 

This  concentration  of  hereditary  characters  is  most  dis- 
tinctly seen  in  the  sexual  cells.  Here  the  other  functions 
are  reduced  to  a  minimum.  The  nucleus  dominates  com- 
pletely. In  the  male  sperms  the  activity  of  the  proto- 
plasm is  limited  to  moving  around  and  to  seeking  the  fe- 
male cells.  The  body  is  made  up  almost  entirely  of  the 
nucleus.  In  the  higher  plants  the  spermatozoids  lack  even 
the  organs  of  free  motion;  they  are  carried  to  the  egg- 
cell  passively,  in  the  pollen-tubes.  The  egg-cells  are  us- 
ually immovable  and  heavy  in  comparison  with  the  male 
elements,  since  they  contain  the  food  substance  necessary 
for  the  incipient  growth  of  the  germ,  and  for  the  first 
cell-divisions. 

Now  fertilization  consists  in  the  union  of  two  cells, 
the  male  spermatozoid  and  the  female  egg-cell.  This 
union  is  the  means  of  combining  the  inheritance  of  the 
two  parents,  and  therefore  the  nuclei  play  the  main  roles. 
The  nucleus  of  the  egg-cell  lies  usually  in  its  center;  the 
male  nucleus  reaches  it  by  passing  straight  through  the 
surrounding  plasm.  Sometimes  one  sees  quite  distinctly 
that  it  no  longer  needs  its  own  protoplasm  since  it  strips 
it  off  and  leaves  it  at  the  border  of  the  egg-cell.  In  the 
Cycadaceae,  in  which  the  spermatozoa  are  just  large 


The  Essence  of  Fertilization  227 

enough  to  be  discernible  with  the  naked  eye,  the  cyto- 
plasm with  all  its  cilia  remains  in  the  outer  layers  of  the 
egg-cell,  while  only  the  nucleus  penetrates  more  deeply. 
The  beautiful  investigations  of  Webber  and  Ikeno  have 
brought  this  process  to  light. 

Finally  the  two  nuclei  come  into  contact  and  unite  into 
a  single  body.  This  is  the  most  important  moment  of 
fertilization,  the  whole  physiological  process  is  concluded 
by  this  union. 

Let  us  ask  now  what  has  been  achieved  by  it.  Appar- 
ently very  little,  for  the  two  parental  nuclei  are  only 
closely  appressed  to  each  other.  A  penetration  or  fusion 
of  their  substance  does  not  take  place.  They  remain  sep- 
arate in  spite  of  the  union.  With  fertilization  the  life  of 
the  new  germ  begins,  and  in  most  cases  immediately. 
Originally  a  single  cell,  the  germ  soon  divides  into  two 
and  then  into  more  cells.  But  this  beginning  of  the  vege- 
tative life  takes  place  everywhere  before  the  two  parental 
nuclei  have  entered  into  closer  union.  Only  after  the 
first  division  does  the  limit  become  unrecognizable,  the 
contact  of  the  constituent  parts  of  the  male  and  female 
halves  being  now  so  intimate  that  there  is  at  least  the 
appearance  of  a  fusion. 

It  was  the  Belgian  investigator,  van  Beneden,  who  dis- 
covered this  all-controlling  fact.  He  first  observed  the 
independence  of  the  paternal  and  the  maternal  nuclei 
in  the  intestinal  worm,  Ascaris,  then  elsewhere  in  the  ani- 
mal kingdom,  and  immediately  recognized  its  significance. 
Since  life  could  begin  without  fusion  of  the  two  nuclei, 
he  considered  that  such  a  thing  was  not  necessary,  and 
assumed  that  all  through  life  the  two  nuclei  preserve  their 
independence  more  or  less  completely. 

According  to  this  view  the  nuclei  are  double  beings, 


228  Fertilisation  and  Hybridisation 

and  we  thus  find,  in  the  material  bearers  of  the  hereditary 
characters,  the  duality  of  which  Goethe  sang  in  his 
"Spriiche  in  Reimen,"  and  which  the  splittings  of  hybrids 
put  so  clearly  before  our  eyes.  Van  Beneden  chose  the 
name  pronuclei  for  the  male  and  the  female  nuclei  that  are 
thus  united,  and  speaks  of  a  pronucleus  male  and  a  pronu- 
cleus  femelle.  This  designation  has  been  retained  since 
that  time,  and  recommends  itself  especially  for  the  reason 
that  the  union  of  the  two  nuclei  is  usually  simply  called  the 
nucleus  of  the  cell ;  and  this  latter  designation  will  prob- 
ably not  be  changed,  although  the  double  nature  of  the 
nucleus  is  recognized.  Therefore  the  pronuclei  are  the 
entities  that  concern  us ;  the  nuclei  are  really  double  nuclei. 

If  the  border  line  between  the  two  pronuclei  remained 
as  distinct  through  life  as  before  the  first  cleavage  and  at 
the  time  of  it,  van  Beneden's  view  would  hardly  meet  with 
any  difficulty.  But  this  is  not  so.  Gradually  the  line  of 
demarcation  becomes  blurred,  and  in  most  cases  nothing 
more  is  to  be  seen  of  it  in  later  life.  But  the  richness  of 
forms  in  nature  is  fortunately  so  great  that  the  general 
phenomena  in  different  organisms  appear  to  us  with  an 
extremely  varied  distinctness.  And  thus  it  is  also  here. 
In  one  species  the  border  line  of  the  pronunclei  is  lost 
sooner,  in  others  later.  It  is  only  a  case  of  finding  the 
best  illustrations,  that  is,  of  selecting  a  species  in  which 
the  paternal  and  the  maternal  inheritances  remain  longest 
visibly  separate. 

The  discovery  of  such  instances  is  the  great  merit  of 
Riickert  and  Hacker.  In  the  one-eyed  water-flea  of  our 
fresh  waters,  the  well-known  Cyclops  vulgaris,  and  its 
nearest  allies,  they  found  a  group  of  animals  in  which  the 
pronuclei  remained  distinctly  separate  for  a  long  time. 
Sometimes  during  several  consecutive  cell-divisions,  some- 


Autonomy  of  the  Pronuclei  229 

times  for  a  longer  period,  and,  in  the  best  cases,  during 
almost  the  entire  vegetative  life,  the  double  nature  of  the 
nuclei  can  here  be  directly  seen.  What  van  Beneden  con- 
cluded from  the  incipient  stages  was  here  irrefutably 
proven. 

The  double  nature  of  the  nuclei  was  also  demonstrated 
more  or  less  distinctly,  and  during  a  shorter  or  longer  se- 
ries of  cell-divisions,  in  other  cases,  by  other  investiga- 
tors. It  was  observed  in  Toxopneustes  by  Fol,  in  Sire- 
don  by  Kolliker,  in  Artemia  by  Brauer,  in  Myzostoma  by 
Wheeler,  in  the  Axolotl  by  Bellonci.  These  and  numerous 
other  observations  now  place  the  law  quite  beyond  doubt. 
The  independence  or  autonomy  of  the  pronuclei  corre- 
sponds everywhere  with  the  mode  of  union  of  the  visible 
parental  characters  in  the  offspring. 

In  the  snail-genus  Crepidula,  Conklin  recently  discov- 
ered a  case  in  which  the  double  nature  of  the  nuclei  can 
be  demonstrated  perhaps  even  more  clearly  and  easily 
than  in  the  Cyclops.  If  the  nuclei  remain  side  by  side  all 
through  life,  the  question  arises  as  to  how  they  dominate 
together  the  development  of  the  child,  the  unfolding  of 
its  characteristics.  Here,  too,  the  results  of  physiology 
and  of  anatomy  work  beautifully  together,  and  here,  too, 
Goethe's  lines  serve  as  a  guide.  Certain  peculiarities  are 
inherited  from  the  father,  others  from  the  mother.  One 
individual  inherits  them  in  this,  another  in  that  mixture. 
The  inheritance  therefore  consists  of  separate  qualities, 
which  may  be  united  in  various  combinations  in  the  off- 
spring. We  are  taught  the  very  same  thing  by  hybrids, 
especially  in  their  progeny,  and  the  rich  floral  splendor  of 
our  horticultural  plants  shows  us  what  an  endless  number 
of  combination-types  have  already  been  achieved  with 
comparatively  few  characteristics. 


230  Fertilisation  and  Hybridisation 

But  we  shall  not  yet  leave  the  subject  of  the  nuclei. 
The  independence  of  all  the  hidden  potentialities,  which 
in  the  physiological  field  is  most  sharply  defined  in  the 
theory  of  pangenesis,  we  can  of  course  not  hope  to  see 
reflected  in  the  nuclei.  We  must,  at  least  for  the  present, 
be  satisfied  to  find  here  any  independent  parts  in  the  nu- 
clei. 

It  was  well  known  to  the  older  investigators,  and, 
among  botanists,  especially  to  Hofmeister,  that  the  nuclei 
are  not  structureless  formations,  but  that  they  exhibit 
more  or  less  distinctly  certain  internal  organs.  But  only 
about  a  quarter  of  a  century  ago  by  means  of  better 
methods  of  investigation  did  Flemming  in  the  zoological 
field,  and  Strasburger  in  the  botanical,  succeed  in  getting 
a  deeper  insight  into  this  structure,  and  soon  afterwards 
Roux  showed  how  these  achievements  are  entirely  in  har- 
mony with  the  requirements  of  the  theory  of  heredity. 
Since  then,  numerous  investigations  have  confirmed  and 
extended  these  results,  and  especially  has  Boveri  brought 
out  the  main  features  in  the  wide  range  of  phenomena. 
To  him  we  owe  the  principle  of  the  independence  of  the 
individual  visible  component  parts  of  the  nuclei,  a  princi- 
ple, which,  in  spite  of  much  opposition,  is  more  and  more 
strongly  supported,  and  which  has  found  in  the  most  re- 
cent studies  of  Sutton  a  brilliant  confirmation. 

What  Boveri's  theory  offers  us  is,  in  the  main  points, 
as  follows :  All  the  bearers  of  hereditary  characters  lie  in 
the  protoplasm  of  the  nucleus,  in  the  nuclear  sap,  as  it  is 
usually  called,  as  definite  particles,  which  can  be  brought 
out  by  various  methods  as  distinctly  recognizable  parts, 
and  which  are  combined  into  threads.  It  is  true  that  one 
cannot  see  the  individual  bearers,  because  there  are  too 
many  of  them  and  they  are  too  small.  Even  a  counting  of 


B oven's  Theory  231 

the  smallest  visible  granules  succeeds  only  rarely.  In  the 
nuclei  of  an  American  salamander,  Batrachoseps,  the 
members  of  the  nuclear  threads  are  most  distinct ;  at  least 
Gustav  Eisen  succeeded  in  making  an  approximate  count 
of  the  smallest  visible  granules.  In  every  pronucleus  they 
form  12  chief  parts,  the  so-called  chromosomes.  Every 
chromosome  showed  as  a  rule  a  subdivision  into  six  sec- 
tions or  chromomeres,  and  every  chromomere,  in  turn, 
appears  again  to  be  built  up  of  six  smallest  granules,  the 
chromioles.  All  in  all  there  are  here  then  about  400  dis- 
tinguishable particles  in  the  individual  pronucleus.  The 
number  of  hereditary  characters  must  certainly  be  much 
higher  than  400  for  such  an  organism;  it  would  more 
likely  have  to  be  estimated  at  ten  times  that  value.  We 
must  therefore  be  satisfied,  for  the  present,  with  the  ob- 
servation of  groups  of  units  in  the  nuclei.3 

In  the  end  there  will  surely  be  found  a  way  of  seeing 
the  individual  units  also.  But  the  resolving  power  of  our 
microscope  will  finally  reach  its  limit,  and  we  shall  prob- 
ably never  be  able  to  see  much  smaller  granulations  than 
the  smallest  elements  that  are  visible  now.  So  far,  even 
the  causes  of  many  contagious  diseases,  in  plants  as  well 
as  in  animals,  are  still  quite  invisible.  But  the  calculations 
which  Errera  has  lately  made  on  the  limits  of  the  smallness 
of  organisms  still  allow  us  full  play.  In  Micrococcus  he 
finds  a  structure  composed  of  about  30,000  protein  mole-, 
cules,  but  many  nuclei  are  much  larger.  It  cannot  yet  be 
estimated  of  how  many  molecules  a  whole  nuclear  thread 
is  composed,  but  it  may  be  assumed  with  certainty  that  not 
every  one  of  its  granules  has  such  a  complicated  structure 
that  it  could  hold  the  factors  for  all  peculiarities  of  the 

3Cf.     Translator's  Preface,  p.  viii. 


232  Fertilisation  and  Hybridisation 

whole  organism.  Their  smallness  would  rather  lead  us  to 
suppose  that  every  one  of  them  could,  at  the  most,  repre- 
sent only  a  small  group  of  such  units. 

To  prove  this,  on  the  one  hand  microscopically,  on  the 
other  hand  experimentally,  is  the  task  that  Boveri  set  for 
himself. 

The  filamentous  framework  in  most  nuclei,  recogniz- 
able by  certain  staining  methods,  is  now  admitted  by  all 
investigators  as  the  idioplasm,  the  bearer  of  the  hereditary 
qualities.  This  thread  is  very  delicate,  and  seems  to  form 
a  skein.  But  when  the  nucleus  prepares  to  divide,  the 
thread  contracts,  and  thereby  is  seen,  what  had  hitherto 
been  invisible,  that  it  is  composed  of  several  separate 
threads.  In  the  nucleus  there  are  several  threads  and  not 
one  single  one.  When  the  contraction  of  the  thread  is  ad- 
vanced so  far  that  the  individual  parts  have  become  quite 
short  and  thick,  they  are  called  chromosomes.  In  the 
nuclei  of  the  body-cells  these  always  occur  in  an  even 
number,  one-half  belonging  to  the  paternal,  the  other  to 
the  maternal  pronucleus. 

In  a  series  of  classical  investigations  Boveri  succeeded 
in  showing  that  the  individual  chromosomes,  on  elongat- 
ing again,  when  the  division  is  accomplished,  retain  their 
independence.  They  remain  the  same  during  their  whole 
life,  elongating  and  shortening  alternately  throughout 
their  entire  development.  The  purpose  of  the  shortening 
is  to  make  possible  an  even  division  of  all  parts  during 
cell-division;  the  threads  then  split  lengthwise,  in  such  a 
way  that  every  single  bearer  of  heredity  first  doubles,  and 
then  sends  the  two  halves  into  the  daughter-nuclei.  This, 
of  course,  could  hardly  be  accomplished  in  a  skein.  On 
the  other  hand  elongation  has  for  its  object  the  freeing  of 
the  bearers  of  heredity  from  that  crowded  accumulation, 


Significance  of  the  Nuclear  Skein  233 

their  task  being  to  control  and  to  direct  the  life  functions 
of  the  cell,  and  to  that  end  they  must  be  able  to  enter  into 
as  free  a  contact  as  possible  with  the  granular  plasm.  An 
arrangement  in  rows,  at  least  of  ,those  bearers  that  are  to 
become  active,  is  the  necessary  condition  thereto,  and  it 
is  evidently  reached  by  means  of  the  elongation  of  the 
threads  and  the  formation  of  the  skein. 

In  order  to  make  possible  an  orderly  retreat  of  the 
individual  threads  out  of  the  tangle  of  the  skein,  every 
thread  is  firmly  attached  by  one  end  to  the  nuclear  wall. 
It  retreats  to  this  point,  which  is  at  the  same  time  the  point 
at  which  its  two  halves,  during  cell-division,  are  pulled 
apart  after  the  splitting.  The  whole  regularity  of  the 
process  would  be  hard  to  explain  without  this  firm  im- 
plantation of  the  individual  nuclear  threads,  as  demon- 
strated by  Boveri.  Where  the  nuclei  are  sinuate  and  the 
nuclear  threads  are  attached  in  the  individual  curves,  the 
conditions  are  specially  clear. 

In  the  species  of  locust,  Brachystola  magnet,  Sutton 
found  the  same  implantations  of  the  nuclear  threads  on 
the  curves  of  the  nucleus.  But  here  every  thread,  of 
which  there  are  eleven  in  every  pronucleus,  forms  a  skein 
after  the  cell-division.  These  skeins  of  one  and  the  same 
nucleus  remain  separated  from  each  other  for  a  long  time, 
and  the  independence  of  the  chromosomes  can  hence  be 
directly  demonstrated,  even  at  the  stage  of  the  skein.  This 
locust  has  also  proven  very  instructive  in  another  point 
of  Button's  studies. 

In  general,  one  finds  the  individual  chromosomes  to  be 
of  unequal  length  in  the  most  various  nuclei.  But,  in  the 
species  of  locust  mentioned,  this  length  occurs  in  such  a 
characteristic  manner  that  the  chromosomes  can  be  easily 
recognized  in  the  successive  cell-divisions.  The  pictures 


234  Fertilisation  and  Hybridisation 

taken  at  the  successive  stages  allow  one  to  follow  up,  with- 
out difficulty,  the  identity  of  the  short  and  thick  nuclear 
threads.  In  doing  so  one  sees  that,  in  the  double  nuclei, 
the  nuclear  threads  lie  in  pairs,  that  is,  that  there  are  two 
nuclear  threads  of  each  individual  length.  Evidently 
these  belong  together  in  such  a  manner,  that  in  every  pair 
one  thread  belongs  to  the  paternal  and  one  to  the  maternal 
pronucleus.  A  border  line  between  them  is  nowhere  to  be 
seen,  and  yet  their  independence  is  very  evident.  And 
this  harmonizes  with  the  conception,  as  detailed  above, 
that,  according  to  the  species  examined,  this  limit  can  be 
observed  for  a  longer  or  shorter  time. 

Microscopic  examinations  teach  us,  then,  to  recognize 
the  independence  of  the  two  pronuclei,  as  well  as  the 
autonomy  of  the  individual  nuclear  threads  or  chromo- 
somes during  the  development  of  the  entire  body.  The 
agreement  of  this  observation  with  the  phenomena  of 
heredity  may  be  considered  as  fully  established. 

But  it  is  another  question  whether  the  individual  chro- 
mosomes correspond  also  to  special  groups  of  hered- 
itary characters,  or,  in  other  words,  whether  the  bearers 
of  the  latter  are  strictly  localized  in  the  nuclear  threads. 
Obviously,  this  question  can  be  answered  only  physiologi- 
cally. It  amounts  to  a  decision  as  to  whether,  if  definite 
chromosomes,  or  definite  parts  in  them,  as  for  example, 
single  chromomeres  and  chromioles,  were  wanting,  defi- 
nite external  characters  of  the  organism  would  also  be 
lacking.  If  it  were  possible  to  kill  a  nuclear  granule  with- 
out otherwise  injuring  the  germ,  what  would  be  the  con- 
sequences ? 

Engelmann  has  taught  us,  in  his  revolutionizing  in- 
vestigation on  the  activity  of  the  individual  chlorophyll 
grains,  how  the  focal  point  of  a  lens  can  be  moved  over 


The  Role  of  the  Chromosomes  235 

the  field  of  a  microscopic  preparation,  thereby  lighting 
up  quite  small  portions  of  a  cell,  and  how  these  portions 
can  thereby  also  be  heated,  and  in  that  way  killed.  If  a 
part  of  a  nuclear  thread  could  be  killed  in  this  way,  the 
externally  visible  consequences  would  certainly  allow  us 
to  draw  conclusions  on  the  relations  of  this  part  to  the 
hereditary  characters.  Perhaps  an  analaysis  of  heredity 
can  some  day  be  made  by  this  method,  but  the  technique  is 
not  yet  sufficiently  advanced  for  this  purpose. 

However,  there  is  another  means  of  removing  individ- 
ual chromosomes,  and  this  again  we  owe  to  the  classical 
investigations  of  Boveri.  He  found  it  in  abnormal  pro- 
cesses of  fertilization  as  they  occur  at  times  in  eggs  of  sea- 
urchins  and  star-fish,  and  it  can  be  quite  easily  produced 
artificially.  It  would  lead  too  far  from  the  main  question 
to  go  into  details  here.  The  important  point  for  our  pur- 
pose is  that,  by  certain  interferences,  a  fertilization  of  one 
egg  with  two  spermatozoa  can  be  achieved.  This  process 
of  dispermia  leads  in  the  nucleus  of  the  germ,  not  to  a 
double,  but  to  a  triple  number  of  chromosomes.  In  the 
successive  divisions  the  conditions  become  correspondingly 
intricate,  and  almost  any  imaginable  abnormal  number  of 
chromosomes  occurs.  Nevertheless,  the  germs  develop  in 
some  cases,  and  then  show  deviations  from  the  normal 
type  which  allow  a  recognition  of  their  normal  relations 
to  the  structure  of  their  nuclei.  Without  doubt  the  germs 
can,  in  every  case,  develop  only  those  qualities  the  repre- 
sentatives of  which  happened  to  be  preserved  in  their 
nuclei. 

We  shall  leave  the  nuclear  threads,  at  present,  and 
return  to  the  two  pronuclei.  We  saw  them  intimately 
combined  during  the  entire  development  of  the  body. 
Now  the  question  arises  as  to  how  long  this  union  persists. 


236  Fertilisation  and  Hybridisation 

And  since  the  double  nuclei  of  the  body  originated  during 
fertilization,  it  is  evident  that  the  conjugating  cells  must 
have  single  nuclei,  and  therefore  that  the  separation  of  the 
pronuclei  must  take  place  at  the  origination  of  these  cells. 

This  fact  is  now  so  generally  established,  for  animals 
as  well  as  plants,  that  it  may  be  regarded  as  one  of  the 
strongest  foundations  of  the  whole  theory  of  fertilization. 
Wherever  it  is  possible  to  count  the  chromosomes,  we  find 
in  the  somatic  cells  twice  as  many  as  in  the  sexual  cells. 
The  former  contain  double  nuclei,  the  latter  single  nuclei, 
or  pronuclei. 

The  sexual  cells  in  animals  originate  directly  from  the 
somatic  cells,  but  in  plants  there  is  more  or  less  prepara- 
tion. Correspondingly,  the  two  pronuclei  separate  in  ani- 
mals at  the  formation  of  the  egg-  and  sperm-cells,  but  in 
the  case  of  plants  before  that.  In  the  seed-bearing  plants 
it  is  the  period  of  the  origination  of  the  mother-cells  of  the 
pollen  and  of  the  embryo-sacs.  Therefore  all  cell-genera- 
tions which  appear  after  this  moment,  and  up  to  the  final 
production  of  the  egg-cells  in  the  embryo-sac,  and  of  the 
sperm-cells  in  the  pollen-grains  and  their  tubes,  possess 
only  pronuclei.  Such  cells  are  called  sexual,  and  the 
period  of  their  formation  the  sexual  generation.  In  ferns 
the  entire  life-period  of  the  prothallium  lies  between  the 
origination  of  the  sexual  cells  and  the  appearance  of  the 
egg-  and  sperm-cells.  This  small  plantlet,  though  built  up 
of  hundreds  of  cells  possesses,  therefore,  as  Strasburger 
has  demonstrated,  only  pronuclei.  The  alternation  of  the 
sexual  prothallia  and  the  asexual  fern-plant  is  called  the 
alternation  of  generations ;  the  two  generations  are  hence 
distinguished  from  each  other  fundamentally  by  their 
nuclei,  which  in  the  leafy  plants  are  always  double  nuclei, 
and  in  the  prothallia  always  pronuclei.  This  difference 


Numerical  Reduction  of  Chromosomes         237 

is  so  constant  that  one  feels  almost  inclined  to  call  the  pro- 
nuclei  prothallial  nuclei. 

At  the  moment  when  the  two  prounclei  separate,  single 
nuclei  appear  in  place  of  the  double  nuclei,  and  the  double 
number  of  nuclear  threads  is  thereby  reduced  to  a  single 
one.  This  process  is  usually  called  the  numerical  reduc- 
tion of  the  chromosomes ;  but  this  imposing  name  means 
nothing  but  the  separation  of  two  nuclei  which  had  so  far 
worked  together  for  a  period.  It  is  like  the  parting  of 
two  persons  who  have  walked  along  together  for  a  while, 
and  will  be  looking  for  other  companionship  presently. 
And  this  they  achieve  by  fertilization. 

This  parting  has  been  minutely  studied  by  numerous 
investigators.  It  has  the  appearance  of  a  nuclear  division 
of  a  very  special  nature,  and  is  frequently  called  the  reduc- 
tion-division, or  heterotypic  nuclear  division.  It  is  neces- 
sarily accompanied  by  a  cell-division,  since  the  two  sepa- 
rated pronuclei  can  only  part  in  separate  cells,  but  this 
cell-division  does  not  always  follow  immediately,  but 
only  after  a  second  essentially  normal  division  of  the 
nuclei.  There  result,  in  that  case,  four  sister-cells  instead 
of  the  usual  two. 

Shortly  before  their  separation,  the  chromosomes  lie 
together  in  pairs,  always  one  in  the  paternal  pronucleus 
united  with  the  corresponding  thread  of  the  maternal 
pronucleus.  They  are  placed  lengthwise  side  by  side. 
Hence  the  separation  evidently  occurs  by  a  longitudinal 
line,  and,  in  by  far  the  greatest  number  of  cases,  this  so- 
called  longitudinal  splitting  of  the  chromosome-pairs  has 
been  observed  in  the  origination  of  the  prouclei.  It  is 
true  that  this  does  not  always  succeed  at  a  first  glance, 
and  it  is  right  here  that  the  differences  of  opinion  between 
different  investigators  have  blurred  the  picture  for  a  long 


238  Fertilisation  and  Hybridization 

time.  But  gradually  it  was  discovered  that  there  are  a 
number  of  secondary  details  which  may  obscure  the  main 
features,  and  we  owe  it  chiefly  to  Strasburger  that  the 
latter  stand  out  clearly  in  the  plant-kingdom.  In  the  ani- 
mal kingdom,  however,  there  is  still  a  series  of  cases 
which  do  not  follow  this  rule,  and  where  the  chromo- 
somes of  the  pronuclei 'are  not  placed  lengthwise  side  by 
side  at  the  moment  of  separation,  but  are  connected  at 
one  end.  Hence  the  separation  here  takes  the  form  of  a 
transverse  division.  Some  insects  and  fresh-water  crabs, 
some  molluscs  and  worms  offer  the  best  known  instances, 
but  according  to  the  most  recent  studies  of  de  Sinety,  Can- 
non, and  others,  the  assumption  gains  ground  that  here  too 
the  microscopic  pictures,  on  closer  observation,  disclose 
a  better  fitting  into  the  otherwise  general  scheme.  It  is 
also  possible  that,  after  the  longitudinal  splitting,  the 
nuclear  threads  still  remain  connected  for  a  while  by  their 
ends,  before  they  finally  separate. 

The  male  and  the  female  sexual  cells  usually  originate 
in  separate  organs,  frequently  on  special  individuals.  This 
goes  to  show  that,  at  their  origination  from  the  body-cells, 
the  paternal  pronuclei  do  not  become  sperms  and  the  ma- 
ternal ones  egg-cells.  On  the  contrary,  the  two  pro- 
nuclei  of  a  mother-cell  in  the  ovary  can  become  egg-cells, 
and  the  two  pronuclei  of  a  pollen  mother-cell  can  both 
give  rise,  by  further  splitting,  to  the  formation  of  sper- 
matozoids.  Accordingly,  one-half  of  the  forming  sperms 
gets  paternal  or  now  grand-paternal  pronuclei,  and  the 
other  half  grand-maternal.  The  same  is  true  of  the 
egg-cells,  and  this  holds  good  in  spite  of  the  circum- 
stance that,  in  consequence  of  the  crowded  condition  of 
the  ovaries,  the  larger  part  of  the  female  cells  has  regu- 


Transmission  of  Grandparental  Characters       239 

larly  to  be  sacrificed  every  time.2  Therefore  fertiliza- 
tion may  result  in  offspring  with  pronuclei  from  the 
grandfather  or  grandmother  only,  or  from  both.  This 
circumstance  may  not  be  without  significance  in  consid- 
ering the  resemblance  between  grandparents  and  grand- 
children among  men. 

But  it  is  not  by  any  means  decisive;  daily  experience 
teaches  that  not  only  in  a  part  of  the  progeny,  but  doubt- 
less in  all  the  offspring,  there  may  be  an  admixture  of  the 
characters  of  the  grand-parents  also.  This  indicates  that 
the  separation  of  the  pronuclei  is  not  of  as  simple  a  nature 
as  the  microscopic  pictures  might  lead  one  to  believe. 
Another  process,  which,  until  now,  has  defied  detection, 
must  take  place,  probably  in  the  smallest,  but  to  us  invisi- 
ble granules  of  the  nuclear  threads.  That  this  is  the 
case  we  learn  especially  from  the  processes  in  hybrids 
and  their  propagation.  Here,  splittings  and  new  combin- 
ations of  the  characteristics  of  the  grand-parents  occur 
in  apparently  incalculable  numbers,  and  here  it  is  dis- 
tinctly seen  that  the  pronuclei  do  not  separate  without 
a  lasting  reciprocal  influence. 

We  shall  first  try  to  get  a  conception  of  this  influ- 
ence, for  the  facts  concerning  hybridization  are  rather 
involved;  they  can  be  most  clearly  explained  by  means 
of  such  a  hypothetical  conception.  We  shall  accordingly 
assume  a  mutual  influence  as  an  established  fact,  and  in- 
quire how  this  can  take  place. 

First  of  all  it  is  clear  that  it  must  be  finished  before 
the  separation  of  the  pronuclei.  Once  they  are  apart  all 
intimate  relation  between  them  ceases.  They  go  their 
separate  ways,  each  living  for  itself.  Only  in  the  double 

2The  reference  is  to  the  resorption  of  the  sister-cells  (when  such 
occur)  of  the  embryo-sac  mother-cell.  Tr. 


240  Fertilisation  and  Hybridization 

nuclei  do  the  paternal  and  the  maternal  pronuclei  lie  so 
close  together  that  their  individual  parts  can  exercise  an 
influence  on  each  other. 

We  have  further  seen  that,  during  the  life  of  a  double 
nucleus,  throughout  the  successive  cell-divisions,  from 
the  origination  of  the  germ  to  the  complete  formation  of 
the  offspring,  the  contact  of  the  pronuclei  becomes  grad- 
ually more  intimate.  Before  the  first  cell  division  they 
are,  as  a  rule,  still  visibly  separated;  soon  afterwards  the 
border-line  begins  to 'look  more  indistinct,  and,  shortly 
before  the  formation  of  the  sexual  cells,  the  double  na- 
ture is  disclosed  with  certainty  only  in  the  rarest  cases 
by  special  structural  relations.  It  is,  therefore,  clear 
that  their  opportunity  for  mutual  influence  gradually  in- 
creases during  somatic  life.  Perhaps  it  first  occurs  only 
at  the  end,  possibly  even,  only  at  the  moment  immediately 
preceding  their  separation.  A  decision  on  this  point  has 
not  yet  been  reached.*  But  the  above-mentioned  vegetative 
splittings  of  hybrids  indicate  that  the  process  is  deferred 
as  long  as  possible.  It  also  seems  simpler  to  assume  that 
it  occurs  only  in  those  cells  which  actually  lead  to  the 
formation  of  sexual  cells,  because  in  the  leaves,  bark,  and 
other  vegetative  parts  of  the  body,  it  would  evidently  be 
without  significance. 

We  therefore  imagine  the  mutual  influence  to  be  exer- 
cised towards  the  end,  or  even  at  the  very  last  moment 
before  the  separation  of  the  pronuclei.  In  the  first  case 


4More  recent  investigations  indicate  that  the  fusion  of  the  male 
and  female  chromatin  elements  is  completed  during  the  stage  known 
as  "synapsis"  which  immediately  precedes  the  reduction-division,  or 
heterotypic  nuclear  division,  referred  to  above.  During  synapsis  the 
chromatin  is  aggregated  into  a  compact  mass  within  the  nuclear 
cavity.  Tr. 


A  Special  Unit  for  Each  Character  241 

it  could  extend  over  a  long  time ;  in  the  latter  it  must  take 
place  suddenly.  In  the  first  case  the  individual  parts  of 
the  nuclear  threads  could  be  mated  one  by  one;  in  the 
latter  this  would  have  to  take  place  everywhere  simultan- 
eously. 

How  this  process  comes  about  is  self-evident  when  we 
assume  special  units,  special  granules  in  the  nuclear 
threads,  for  the  visible  characters  of  the  organisms.  There 
must  be  as  many  units  in  the  nucleus,  as  a  plant  or  animal 
possesses  individual  characters.  And  this,  of  course,  is 
the  rule  for  both  pronuclei.  In  the  condition  of  the  short 
and  thick  chromosomes  these  units  lie  crowded  together. 
This  is  a  definite  stage  in  cell-division ;  the  units,  at  least 
those  of  the  interior  of  the  group,  remain  in  a  condition 
of  enforced  rest.  But  as  soon  as  cell-division  is  com- 
pleted, the  nuclear  threads  stretch ;  they  become  quite  long 
and  thin,  and  indeed  so  long  that  a  large  part,  perhaps 
most  of  them,  possibly  all  of  them,  come  to  the  surface. 
At  least  stretched  out  in  a  row  in  this  way,  the  granules 
must  then  be  arranged  one  after  another,  perhaps  in  the 
threads  themselves,  perhaps  in  their  finest  ramifications. 
Now  they  become  active,  and  if,  at  this  time,  nuclear 
threads  of  the  paternal  and  the  maternal  pronuclei  lie 
together  in  pairs,  every  granule  can  enter  into  communion 
with  its  corresponding  unit  in  the  other  pronucleus. 

There  is  no  reason  to  assume  that  the  exceedingly  fine 
structure  of  the  nuclei,  which  is  so  strikingly  to  the  pur- 
pose and  yet  so  simple,  should  be  limited  to  what  is  visible 
to  us  at  present.  On  the  contrary  everything  points  to 
the  probability  that,  in  the  internal  structure  also  of  the 
nuclear  threads  this  same  serviceable  rule  must  prevail. 
The  whole  complicated  process  of  nuclear  division  has 
for  its  object  the  division  of  the  two  pronuclei  in  such  a 


242  Fertilisation  and  Hybridisation. 

way,  that  their  daughter-nuclei  will  share  alike  in  the 
hereditary  characters  that  are  present.  The  lengthen- 
ing of  the  nuclear  threads  at  the  close  of  division,  their 
so  frequent  ramification,  and  the  seemingly  irregular  in- 
tertwining of  their  parts,  evidently  indicates  the  possi- 
bility of  a  domination  of  the  cell-life  by  the  bearers  of 
the  inheritable  qualities.  These  must  impress  their 
character  on  the  surrounding  protoplasm  either  dynami- 
cally or,  as  I  have  assumed  in  my  Intracellulare  Pangen- 
esis,  through  a  giving  out  of  material  particles  to  the 
surrounding  protoplasm,  and  thus  promote  growth  and 
development,  in  the  prescribed  direction,  into  the  specific 
form  of  the  species  to  which  the  organism  belongs. 

This  secretion  of  material  chromatin  particles  from 
the  nuclei  was  recently  demonstrated  by  Conklin  in  Crep- 
idula.5  In  this  way  considerable  quantities  of  chromatin, 
and  therefore  probably  of  pangens  also,  are  transferred 
into  the  somatic  protoplasm. 

Thus  we  consider  that  the  structure  of  the  nuclear 
threads  is  such  that  it  not  only  makes  possible,  but  regu- 
lates and  dominates  the  relations  of  the  two  pronuclei. 
In  an  ordinary  animal,  or  in  a  plant  which  is  not  a  hybrid, 
both  pronuclei  possess  the  same  units,  only  with  a  some- 
what unlike  degree  of  development.  We  assume,  there- 
fore, that  the  cooperation  comes  about  in  such  a  way  that 
the  individual  units  in  the  stretched  threads  lie  in  the 
same  numerical  order.  Then,  when  the  threads  are 
closely  appressed  lengthwise,  in  pairs,  we  can  imagine  that 
all  the  like  units  of  the  two  pronuclei  lie  opposite  each 
other.  And  this  is  obviously  the  simplest  assumption 
for  a  mutual  influence. 

5Strasburger  failed  to  find  any  direct  evidence  of  such  a  transfer 
of  particles  in  plants.  Cf.  the  Translator's  Preface,  p.  viii.  Tr. 


An  Exchange  of  Character-Units  243 

If  every  unit,  that  is,  every  inner  character  or  every 
material  bearer  of  an  external  peculiarity,  forms  an  en- 
tity in  each  pronucleus,  and  if  the  two  like  units  lie  oppo- 
site each  other  at  any  given  moment,  we  may  assume  a 
simple  exchange  of  them.  Not  of  all  (for  that  would 
only  make  the  paternal  pronucleus  into  a  maternal  one), 
but  of  a  larger-,  or  even  only  a  smaller  part.  How  many 
and  which;  may  then  simply  be  left  to  chance.  In  this 
way  all  kinds  of  new  combinations  of  paternal  and  mater- 
nal units  may  occur  in  the  two  pronuclei,  and  when  these 
separate  at  the  formation  of  the  sexual  cells,  each  of  them 
will  harbor  in  part  paternal,  in  part  maternal  units.  These 
combinations  must  be  governed  by  the  laws  of  proba- 
bility, and  from  these,  calculations  may  be  derived,  which 
may  lead  to  the  explanation  of  the  relations  of  affinity 
between  the  children  and  their  parents,  the  grandchildren 
and  their  grand-parents.  On  the  other  hand  a  compari- 
son of  the  results  of  this  calculation  and  of  direct  obser- 
vation will  form  the  best,  and  for  the  time  being,  the  only 
possible  means  for  a  decision  as  to  the  correctness  of  our 
supposition. 

The  mutual  influence  of  the  two  pronuclei  shortly  be- 
fore their  separation  is  therefore  brought  about,  accord- 
ing to  our  view,  by  an  exchange  of  units.  Every  unit 
can  be  exchanged  only  for  a  like  one,  which  means  for 
one  which,  in  the  other  pronucleus,  represents  the  same 
hereditary  character.  This  rule  appears  to  me  to  be  un- 
avoidable and  really  self-evident.  For  the  children  must 
inherit  all  specific  characters  from  their  parents,  and  they 
must  also  transmit  all  of  them  to  their  own  progeny. 
This  exchange  must  hence  be  accomplished  in  such  a  way 
that  every  pronucleus  retains  the  entire  series  of  units 
of  all  the  specific  characters,  and  this  result  can  evidently 


244  Fertilisation  and  Hybridisation 

be  obtained  only  when  the  interchange  is  limited  to  like 
units. 

We  distinguish  here  specific  characteristics  from  indi- 
vidual features.  The  units  in  the  hereditary  substance 
of  the  nuclear  thread  compose  the  former.  Every  species 
has  an  often  exceedingly  large  and  yet  definite  and  invari- 
able number  of  them.  The  sum  total  of  these  units 
forms  that  which  distinguishes  any  given  species  from  all 
others,  even  from  its  nearest  allies.  A  complete  diagno- 
sis of  a  species  would  have  to  embrace  all  of  these  char- 
acteristics, and  therewith  all  the  material  bearers  under- 
lying them. 

The  individual  features,  that  is,  the  differences  be- 
tween the  individuals  within  the  species,  and  not  only  of 
the  systematic  but  of  the  so-called  elementary  species,  are 
of  quite  another  nature.  It  is  true  that  they  are,  in  a  way, 
hereditary,  but  with  that  they  are  subject  to  constant 
changes.  The  average  stature  of  man  remains  the  same 
in  the  course  of  centuries,  for  the  same  race  (elementary 
species),  but  the  individual  stature  changes  constantly 
from  one  individual  to  another.  In  the  somatic  cells  of 
man  the  bearers  of  the  stature  of  the  father  lie  opposite 
those  of  the  mother.  At  the  moment  of  exchange  these 
are  mutually  transferred,  and  the  sexual  cells  receive 
partly  one,  partly  the  other  stature,  but  this  in  the  most 
various  combinations  with  the  other  characters.  Thus 
one  might  continue.  Every  visible  quality,  every  trait 
of.  character  is  to  be  found  in  all  individuals,  only  in  some 
they  are  strongly  developed  and  prominent,  in  others 
weak  and  recessive.  Ordinary  observation  takes  more 
interest  in  differences  than  in  similarities,  and  for  this 
reason  the  former  are  designated  by  contrasting  expres- 
sions, as  large  and  small,  strong  and  weak,  forward  and 


Individual  Variation  245 

modest.  But  these  are,  in  each  instance,  only  degrees 
of  the  same  hereditary  characteristic,  or  the  same  trait 
of  character.  And  such  more  or  less  differing  stages 
of  development  of  the  same  inner  units  we  represent  to 
ourselves  as  the  entities  which  are  exchanged  by  the  nu- 
clear threads. 

Individual  differences  are  thus  not  included  in  the 
type  of  the  species.  They  form  deviations  from  this 
type,  and  are  conditioned  by  causes  which  were  formerly 
generally  described  as  conditions  of  nutrition,  but  now 
more  frequently  as  environment.  Under  these  influences 
every  character  can  develop  more  or  less  strongly  than 
the  average  type.  And  the  environment,  provided  it  re- 
mains constant  during  the  entire  period  of  development, 
must  affect  all  the  unfolding  characters  in  the  same  way. 
If  it  is  favorable  it  furthers  all  parts  of  the  body  and  all 
mental  gifts,  if  it  is  unfavorable  it  has  the  opposite  effect 
on  all  of  them.  Not,  by  any  means,  to  the  same  degree 
upon  all  of  them :  that  does  not  depend  upon  the  environ- 
ment but  upon  the  units  themselves;  this,  however,  can 
not  lead  to  essential  differences  between  separate  individ- 
uals. But  our  supposition  of  such  a  uniform  environ- 
ment would  probably  be  met  with  only  in  the  rarest  of 
cases.  And,  as  soon  as  it  changed,  it  would  influence 
one  individual  differently  from  the  others.  Moreover 
the  characters  do  not  unfold  simultaneously,  but  success- 
ively, the  higher  ones  for  the  most  part  later  than  the  lower 
ones,  mental  characters  later  than  those  of  the  body,  the 
reason  later  than  the  memory.  And  all  those  wheels 
work  into  each  other  so  that  small  deviations  will  rather 
tend  to  become  greater  than  to  be  equalized.  Though 
children  of  the  same  parents  but  of  different  age  might, 
during  their  entire  youth,  live  under  the  same  circum- 


246  Fertilisation  and  Hybridisation 

stances,  they  will  yet  react  differently  to  them.  This  also 
holds  true  for  plants  where,  in  the  same  bed,  a  delay  of 
only  one  day  in  germinating  will,  according  to  the  weather, 
lead  either  to  equal  or  to  quite  surprising  differences  in  size 
and  qualities. 

If  favorable  and  unfavorable  conditions  of  life  alter- 
nate during  the  individual  development,  and  if  they  strike 
a  group  of  individuals  sprung  from  like  seeds  at  different 
periods  of  their  growth,  quite  a  considerable  degree  of 
individual  differences  must  thereby  result. 

These  differences  play  in  nature  the  same  role  as  in 
human  society.  One  is  adapted  for  this  kind  of  task,  the 
other  for  that.  With  men  it  is  the  duty  of  every  one  to 
develop  his  own  talents  to  the  best  of  his  ability,  and  to 
render  as  favorable  as  possible  the  circumstances  for  the 
most  perfect  development  of  his  children.  The  highest 
efficiency  of  society  in  general  demands  of  each  the 
strongest  effort  in  the  direction  of  his  most  favorable 
talents.  To  ascertain  this  direction  ought  to  be  one  of 
the  chief  aims  of  education  and  instruction.  In  animals 
and  plants  this  highest  efficiency  can  obviously  not  be 
achieved  in  the  same  way.  And  especially  are  the  con- 
ditions different  for  plants,  which  are  tied  for  life  to  the 
place  where  they  germinated.  Here,  as  is  well  known, 
nature  is  assisted  by  the  astonishingly  great  number  of 
seeds ;  she  sows  so  many  in  every  individual  spot  that  only 
the  best,  that  is,  the  individuals  best  adapted  for  the  given 
locality,  need  retain  life.  But,  by  sacrificing  countless 
seeds,  she  also  accomplishes  here  that  adaptation  of  the 
individual  specimens  which  is  the  condition  for  the  com- 
plete unfolding  of  their  abilities  and  advantages. 

Very  great  weight  is  therefore  given  to  individual 
differences  in  the  life  of  the  entire  species.  The  greater 


The  Significance  of  Sexual  Reproduction        247 

they  are,  the  greater  the  power  of  adaptation,  the  greater 
the  chance  of  victory. 

And  in  this  I  see  the  significance  of  sexual  reproduc- 
tion. It  mixes  the  potentialities  that  have  developed  in 
the  single  individuals  in  the  most  complete  mariner  imag- 
inable; it  achieves,  at  one  stroke,  all  possible  combina- 
tions. It  cancels,  as  Johannsen  expresses  it,  the  previous 
correlation's.  Asexual  propagation  confers  a  certain 
degree  of  variability,  and  this  may  be  quite  sufficient  in 
many  cases,  especially  in  the  case  of  a  low  organization 
or  of  quite  special  adaptation,  as  in  many  parasitic  and 
saprophytic  organisms.  Under  such  conditions  the  vari- 
ability remains,  in  a  certain  sense  limited,  more  or  less 
one-sided,  because  every  individual  is  the  result  of  the 
varying,  but,  on  the  whole,  one-sided  environment  in 
which  his  progenitors  existed.  Only  an  exchange  of  qual- 
ities can  help  to  overcome  this  one-sidedness ;  only  this 
can  cause  all  the  combinations  to  arise  which  are  de- 
manded by  the  varying  environments.  If  we  assume  that 
the  bearers  of  the  individual  characters  are,  as  a  rule,  in- 
dependent of  each  other  during  their  exchange,  and  also 
that  the  latter  is  ruled  by  chance,  two  pairs  of  character- 
istics would  directly  result  in  four,  three  in  eight,  four 
in  sixteen  combinations.  The  sum  total  of  the  points  of 
difference  of  two  parents  must  therefore  give  rise  to  such 
an  incredible  number  of  possibilities  that  no  struggle 
for  existence,  no  annual  rejection  of  hundreds  and  thou- 
sands of  germs  could  demand  a  richer  material. 

Hence  sexual  reproduction  brings  individual  variabil- 
ity to  its  highest  point.  It  produces  a  material  that  cor- 
responds to  almost  any  environment.  It  is  the  principal 
condition  for  the  greatest  efficiency  of  cooperation,  be  it 
by  a  selection  as  free  as  possible  of  the  line  of  develop- 


248  Fertilisation  and  Hybridization 

ment  for  the  single  individuals,  or  by  a  sacrifice  of  all 
the  individuals  that  do  not  quite  meet  all  the  requirements. 

This  service  of  sexual  reproduction  is  evidently  not 
limited  to  a  single  generation.  It  exercises  its  influence 
throughout  successive  generations,  and  it  is  probably  in- 
different whether  the  effect  follows  directly,  or  whether  it 
manifests  itself  in  the  course  of  time.  Even  without  that, 
the  complete  utilization  of  all  given  possibilities  requires, 
as  a  rule,  more  individual  beings  than  are  born  in  a  single 
generation.  And  with  this,  the  otherwise  strange  fact  is 
explained,  that  the  exchange  of  the  units  does  not  imme- 
diately follow  fertilization,  but  only  takes  place  a  short 
time  before  the  succeeding  period  of  fertilization.  But 
obviously  an  exchange,  ruled  by  laws  of  chance,  could  not 
benefit  a  given  isolated  individual  or,  more  correctly  speak- 
ing, it  would  most  likely,  just  as  frequently  be  harmful 
as  useful.  It  can  only  be  of  use  in  connection  with  an 
increase  in  the  number  of  individuals,  for  it  is  its  task  to 
bring  about  as  great  a  variety  as  possible,  and  with  that, 
the  highest  possible  prospect  for  the  required  quantity 
of  superior  specimens.  At  the  moment  when  the  produc- 
tion of  the  sexual  cells  begins,  in  such  enormous  numbers, 
it  also  finds  the  best  opportunity  for  fulfilling  its  task. 

Thus,  sexual  reproduction  has  only  a  subordinate  sig- 
nificance for  the  children,  while  for  the  grandchildren  it 
is  of  the  utmost  importance,  because  only  for  them  does 
the  urn  mix  up  all  its  lots. 

The  same  laws  that  govern  normal  fertilization,  are, 
of  course,  valid  for  hybrids  also.  There  cannot  be  special 
biological  laws  for  them,  because  they  are  only  derived 
phenomena,  deviations  from  the  normal.  Now  the  ques- 
tion is,  to  which  results,  departing  from  the  rule,  will  the 
common  laws  lead  in  these  special  cases.  And  with  this 


Hybrids,  Varieties,  and  Species  249 

it  is  clear  that  the  phenomena  must  keep  nearer  to  the 
normal  the  less  the  deviation  is  from  the  type. 

This  type  is  conditioned  by  the  fact  that  the  two  or- 
ganisms that  fertilize  each  other  belong  to  the  same  small 
or  elementary  species.  They  have  then,  on  the  whole, 
the  same  characters,  even  if  these  are,  according  to  their 
environment  in  various  degrees  of  development.  There 
are  no  differences  among  them  independent  of  this,  at 
least  if  we  consider  the  cumulative  effect  of  uniform  in- 
fluences in  the  course  of  several  generations. 

As  soon  as  such  independent  differences  occur,  and  as 
soon  therefore  as  there  are  present  constant  contrasts, 
which  are  retained  in  the  sequence  of  generations  and 
cannot  be  blended  by  environment,  we  call  the  sexual 
union  of  two  individuals  a  crossing  or  a  hybridization. 
If  the  contrasts  are  slight,  we  call  the  two  races  varieties, 
if  they  are  greater,  they  assume  the  rank  of  species.  The 
crossing  of  varieties  keeps  quite  near  to  normal  fertiliza- 
tion ;  that  of  the  species  deviates  the  more  the  slighter  the 
relationship  between  them.  The  crossing  of  varieties 
forms  a  type  complete  in  itself,  that  of  the  species  forms 
a  series  which  descends  from  almost  normal  processes, 
by  gradual  progress,  to  a  complete  reciprocal  sterility. 
The  variety-hybrids  are  fertile  like  their  parents,  but  in 
the  species-hybrids  the  diminished  fertility  indicates  ab- 
normal phenomena  either  in  fertilization  or  in  the  ex- 
change of  the  units. 

We  must  therefore  discuss  these  two  groups  sep- 
arately, and  we  shall  begin  with  the  varieties. 

In  daily  life  and  in  horticulture,  any  thing  that  deviates 
from  the  normal  is  called  a  variety.  Even  the  new  forms 
obtained  by  crossing  are  quite  commonly  counted  among 
the  varities.  In  science,  therefore,  the  word  would  really 


250  Fertilisation  and  Hybridisation 

be  useless.  Nevertheless  it  has  been  retained  and  its 
meaning  has  been  gradually  limited.  Especially  in  de- 
scribing horticultural  plants  the  conception  is  sufficiently 
restricted  by  excluding  on  the  one  hand  the  hybrids,  on 
the  other  hand  the  improved  races  obtained  by  selection, 
and  finally  the  so-called  elementary  species  that,  taken 
together,  form  our  ordinary  species. 

Upon  reviewing  the  cases  that  are  left,  two  types  can 
be  plainly  distinguished,  the  constant  and  the  inconstant 
varieties.  The  former  are  not  inferior  to  true  species  in 
point  of  constancy.  Their  characters  vary,  in  the  single 
individuals,  around  a  mean,  but  in  the  main  not  more  so 
than  the  corresponding  characteristic  of  the  species. 
From  this  they  are  separated  by  a  decided  chasm.  In 
pure  fertilization  they  never  bridge  this  chasm,  or  at 
least,  extremely  rarely,  but  in  crossing  they  revert  very 
easily  to  the  species.  It  is  this  very  reversion  that  stamps 
them  varieties,  and  when  the  crossing  is  not  artificial  but 
natural,  brought  about  by  insects,  it  escapes  observation, 
and  only  the  fact  of  the  reversion  strikes  the  gardener. 

These  constant  varieties  are,  as  a  rule,  distinguished 
from  the  species  to  which  they  belong,  by  lacking  some 
striking  quality  that  adorns  the  latter.  Most  frequently 
it  is  the  coloring  of  the  flower  or,  in  the  case  of  flowers 
with  combined  colors,  as  in  the  yellow  and  red  tulips,  one 
of  the  individual  colors,  that  is  wanting.  Often  they 
lack  hairs  or  thorns,  very  frequently  the  development  of 
the  blade  is  arrested,  and  split  leaves  originate.  In  all  of 
these  cases  there  is  no  ground  for  the  opinion  that  the 
failure  of  the  visible  character  means  also  the  loss  of  the 
respective  unit.  Rather  does  everything  point  to  the 
fact  that  the  unit  has  simply  become  inactive,  that  it  is  in 
a  state  of  rest,  or  as  it  is  usually  expressed,  that  it  has  be- 


Inconstant  Varieties  251 

come  latent.  Especially  the  reversions,  which  in  individ- 
ual specimens  of  such  varieties  are,  at  times,  quite  com- 
mon phenomena,  betray  this  latent  presence. 

Inconstant  varieties  are  distinguished  by  a  strikingly 
high  variability,  by  an  exceedingly  great  range  of  depart- 
ure from  the  norm.  But  here  we  encounter  the  double 
meaning  of  the  designation  inconstancy.  On  the  one 
hand  the  word  means  a  certain  relatively  great  richness 
of  individual  forms,  on  the  other  hand  it  relates  to  differ- 
ences between  the  parents  and  the  progeny.  In  choosing 
from  an  inconstant  variety  a  single  individual,  and  sowing 
its  seed,  after  pure  fertilization,  the  whole  play  of  forms 
of  the  variety  can  be  found  again  in  the  children, — hence 
a  palpable  proof  of  the  inconstancy.  But,  on  choosing 
several  individuals,  and  on  sowing  their  seeds  separately, 
each  of  them  will  produce  almost  the  same  series  of  forms. 
The  whole  group  is  transmitted  from  year  to  year,  and 
does  not  change.  The  variety  has  a  definite  circle  of 
forms  in  which  the  descendants  of  every  specimen  choose 
freely  their  place,  but  they  do  not  go  outside  the  circle. 
The  limits  are  constant,  and  remain  so  in  the  course  of 
generations ;  within  the  limits,  however,  a  motley  variety 
prevails. 

Such  is  the  concept  of  plants  with  variegated  leaves, 
of  double  and  striped  flowers,  and  many  other  most  highly 
variable  garden-plants.  The  new  character  is  not  based 
here  on  the  loss  or  the  latency  of  some  characteristic  of 
the  species.  Indeed,  on  the  contrary,  it  is  usually  a  pecu- 
liarity which  is  already  present  in  the  species  itself,  or  at 
least  in  one  of  its  races,  in  a  latent  state.  Especially  do 
variegated  leaves  occur,  not  so  very  infrequently,  on 
otherwise  green  plants,  and  the  same  is.  true  of  stamens 
with  petal-like  broadenings.  The  relation  of  the  incon- 


252  Fertilisation  and  Hybridisation 

stant  varieties  to  the  species  from  which  they  are  derived, 
is  therefore  quite  different  from  that  of  the  constant 
varieties. 

Nevertheless,  the  two  crossings  behave  in  the  same 
manner  in  regard  to  their  mother-species.  From  the  lat- 
ter they  are  distinguished,  for  the  most  part,  only  in  one 
point,  though  sometimes  in  several.  But  we  have  always 
to  deal  with  the  distinction  between  active  as  contrasted 
with  latent,  be  it  that  the  given  character  is  active  in  the 
variety  and  latent  in  the  mother-species,  or  latent  in  the 
former  and  active  in  the  species  itself. 

If  to  this  we  apply  the  conception  of  the  arrangement 
of  the  units  in  rows  on  the  nuclear  threads,  as  explained 
above,  it  is  quite  evident  that  everything  will  follow  ex- 
actly the  same  course  as  in  normal  fertilization.  Every 
unit  in  the  paternal  pronucleus  corresponds  to  the  repre- 
sentative of  the  same  peculiarity  in  the  maternal  one. 
The  nuclear  threads  fit  as  nicely  into  each  other  as  in  a 
pure  species,  and  all  the  units  which  do  not  directly  bring 
about  the  point  of  difference  behave  quite  normally.  Co- 
operation in  vegetative  life,  and  exchange  during  the 
formation  of  the  sexual  cells  need  not  be  disturbed.  We 
may  confine  our  whole  consideration  to  the  point  of  dif- 
ference, and  we  shall  select,  for  the  purpose,  as  simple 
an  illustration  as  possible,  one  in  which  there  is  only  one 
difference  between  the  species  and  the  variety,  for  exam- 
ple, the  color  of  the  flower. 

The  material  bearer  of  the  color-characteristic  is  situ- 
ated in  the  mother-species  so  that  it  can  display  its  full 
activity  while  in  the  variety  it  is  unable  to  do  so.  If  the 
paternal  and  maternal  nuclear  threads  of  the  hybrid  come 
into  contact  for  the  purpose  of  exchange,  and  with  the 
same  sequence  of  units  in  both,  the  active  unit  of  coloring 


First  vs.  Second  Hybrid-Generation  253 

matter  naturally  gets  the  equivalent  inactive  unit  as  an 
antagonist.  With  this  it  must  therefore  be  exchanged. 
We  assume  that  in  this  the  latent  condition  is  without 
significance,  that  hence  the  exchange  comes  about  in  the 
same  manner  as  in  normal  fertilization. 

Over  this,  however,  the  crossings  of  varieties  have  the 
great  advantage  that  there  the  origin  of  the  characteris- 
tic 'in  question  can  always  be  clearly  and  positively  rec- 
ognized. Both  units  of  a  pair  of  antagonists  are  other- 
wise distinguished  only  by  a  more  or  less  of  development, 
here  by  a  sharp  contrast.  And  for  this  reason  it  is  experi- 
mentally much  easier  to  discover  the  laws  with  varieties 
than  with  purely  individual  differences. 

In  doing  this,  two  points  have  to  be  distinguished ;  the 
consequences  of  fertilization  and  the  consequences  of  the 
exchange  of  the  units.  The  former  we  see  in  the  hybrid 
itself,  the  latter  in  its  descendants.6  And  since  fertiliza- 
tion and  exchange  are  two  such  fundamentally  different 
things,  we  must  not  wonder  that  there  exist  such  decided 
differences  between  a  hybrid  and  its  descendants.  These 
differences  show  themselves  essentially  by  the  fact  that 
the  hybrids  of  a  mother-species  with  a  variety  of  the  same 
are  alike,  even  if  they  are  obtained  in  great  numbers, 
while  their  descendants  always  display  a  certain  variety. 

Let  us  first  consider  the  first  generation  of  variety- 
hybrids.  How  do  the  two  pronuclei,  notwithstanding 

6In  the  fertilized  egg,  resulting  from  the  crossing,  the  chromatin 
from  the  male  and  female  parents  is  not  completely  fused.  As  pointed 
out  in  a  preceding  footnote  (p.  240),  this  fusion,  called  synapsis, 
occurs  as  almost  the  last  step  preceding  the  nuclear  and  cell-divisions 
that  give  rise  to  the  reproductive  cells.  The  characters  of  the  first 
hybrid  generation  are  a  result  of  fertilization.  Following  synapsis, 
the  pure  bred  offspring  of  this  generation  differ  from  their  parents 
and  also  among  themselves.  Tr. 


254  Fertilisation  and  Hybridisation 

their  inequality,  cooperate  in  order  to  regulate  the  evolu- 
tion ?  This  question  amounts  to  the  same  as  asking,  what 
is  the  sum  of  the  influence  of  an  active  and  a  latent  unit  ? 
At  first  glance  one  would  expect  that  this  influence  would 
correspond  to  half  the  value  of  a  pair  composed  of  two 
active  units.  Previously  this  opinion  was  rather  gener- 
ally accepted,  and  there  was  an  inclination  to  regard  plants 
with  intermediate  characters  as  hybrids.  Especially  many 
plants  with  pale  red  or  pale  blue  flowers  were  regarded 
as  such.  But  the  experience  of  later  years  has  decided 
differently. 

Variety-hybrids  generally  bear  the  characteristic  of 
the  species,  sometimes  fully  developed,  sometimes  more 
or  less  weakened^  but  this  for  the  most  part  only  so  little 
that  superficial  observation  sees  no  difference.  An  active 
and  a  latent  unit  are  not  essentially  different  in  their  co- 
operation from  two  active  ones ;  a  fact  which  may  prob- 
ably be  best  explained  by  the  assumption  that  two  cannot 
accomplish  more  than  one  already  does.  This  conception 
finds  a  very  strong  support  in  the  results  of  the  most 
recent  investigations  by  Boveri  on  dispermia,  which  we 
have  already  partly  discussed.  By  fertilizing  one  egg 
with  two  spermatozoa  the  composition  of  the  structure 
of  the  nuclear  threads  can  be  altered  in  different  ways, 
for  instance,  in  such  a  manner  that  in  one  nucleus  there 
lie  not  two,  but  three  pieces  of  any  one  of  its  chromo- 
somes. It  might  then  be  expected  that  the  given  charac- 
ters would  be  very  strongly  developed,  to  about  one  and 
one-half  of  their  intensity.  But,  as  far  as  can  be  judged 
from  Boveri 's  experiments,  this  is  not  the  case,  and  the 
influence  of  the  three  equivalent  units  is  not  noticeably 
greater  than  that  of  two. 

We  come  now  to  the  progeny  of  hybrids,  and  we,  of 


Disjunction  of  Hybrids  255 

course,  presuppose  self-fertilization.  At  the  formation 
of  the  sexual  cells  the  two  pronuclei  separate ;  this  happens 
at  the  origination  of  the  egg-cells  as  well  as  of  the  sperms. 
Through  exchange,  the  active  units  of  our  differing  pair 
combine  partly  with  new  units  of  the  other  pairs,  and 
thereby  new  combinations  originate  as  in  ordinary  fertili- 
zation. But  if  we  consider  only  the  differing  pair,  exactly 
one-half  of  the  egg-cells  must  obviously  have  the  pater- 
nal, and  the  other  half  the  maternal  character.  Or,  in 
other  words,  in  one-half  of  the  egg-cells  the  given  charac- 
ter occurs  in  the  active,  in  the  other  in  the  latent  state. 
Exactly  the  same  is  true  of  the  male  sexual  cells,  the 
sperms,  in  animals  as  well  as  in  plants,  and  independently 
from  the  circumstance  that  in  the  higher  plants  the  sperm- 
cells  are  conducted  to  the  egg-cells  in  the  pollen-tube. 

The  male  sexual  products  of  a  hybrid  are  therefore 
unlike  each  other,  and  the  same  holds  true  of  the  female. 
In  the  simplest  case  selected  both  groups  consist  of  two 
types,  in  the  more  complicated  cases  this  number  will  ob- 
viously become  greater.  The  paternal  and  maternal  fac- 
tors of  the  hybrid  become,  in  its  progeny,  grandpaternal 
and  grandmaternal.  Hence,  in  regard  to  the  point  of 
difference,  one-half  of  its  egg-cells  and  one-half  of  its 
sperm-cells  have  grandpaternal  factors,  while  the  other 
halves  possess  grandmaternal  ones. 

By  means  of  this  principle  the  composition  of  the  pro- 
geny in  the  simple  as  well  as  in  the  complex  cases,  and  for 
constant  as  well  as  for  inconstant  varieties  can  be  calcu- 
lated. Thus  we  obtain  the  formulae  which  are  now  uni- 
versally known  as  MendeTs  law. 

They  indicate,  foranygiver!*  number  of  points  of  dif- 
ference between  two  parents,  how  many  children  corres- 
pond to  every  individual  combination  of  the  respective 


256  Fertilisation  and  Hybridisation 

character.  And,  on  the  whole,  experience  has  so  far 
proven  the  reliabilty  of  these  formulae  for  animals  as  well 
as  for  plants. 

It  would  be  too  great  a  digression  to  consider  here  the 
formulae  themselves.  We  shall  therefore  leave  the  field 
of  the  variety-hybrids,  and  turn  to  the  hybrids  between 
different  species,  especially  between  allied  elementary  spe- 
cies. 

In  order  to  understand  these  we  must  get  a  clear  idea 
of  the  nature  of  the  points  of  difference  in  this  case,  or  in 
other  words,  what  is  meant  by  relationship.  Species  orig- 
inate from  each  other  in  a  progressive  way.  The  number 
of  the  units  in  lower  organisms  is  evidently  only  small, 
and  must  gradually  increase  with  progressing  organiza- 
tion. Every  newly  arising  species  contains  at  least  one 
more  than  the  form  from  which  it  has  arisen.  Only  in 
this  way  can  one  imagine  the  progress  of  the  entire  plant 
and  animal  world.7 

It  is  indeed  questionable  whether  the  acquisition  of  a 
single  new  unit,  the  increasing  by  one  unit  of  the  entire 
stock,  amounting  to  hundreds  and  thousands,  would  be 
sufficient  to  make  the  impression  of  progress  on  us.  The 

7A  quite  different  hypothesis  is  thinkable,  as,  for  example,  that 
suggested  by  G.  H.  Shull,  "The  Significance  of  Latent  Characters," 
Science  N.  S.,  25 :  792.  1907. 

"All  the  visible  variations  of  the  present  plant  and  animal  world 
were  once  involved  in  some  generalized  form  or  forms,  and  the  pro- 
cess of  differentiation  pictures  itself  to  us  as  a  true  process  of  evolu- 
tion brought  about  by  the  change  of  individual  character-determining 
units  from  a  dominant  to  a  recessive  state.  This  conception  results 
in  an  interesting  paradox,  namely  the  production  of  a  new  character 
by  the  loss  of  an  old  unit." 

This  hypothesis,  however,  as  de  Vries  has  pointed  out,  seems  too 
much  like  a  revival  of  the  old  evolution  theory  as  opposed  to  epi- 
genesis.  Tr. 


Avuncular y  vs.  Collateral  Crossings  257 

difference  will  in  most  cases  be  too  slight.  Only  when 
two  or  three  or  more  units  have  been  added  successively 
to  those  already  present,  will  we  recognize  an  increase  in 
the  degree  of  organization. 

The  progress  of  every  individual  species  can  appar- 
ently take  different  directions.  In  some  genera  there  are 
species  so  typical  that  they  may  be  regarded  as  the  com- 
mon origin  of  the  others.  Where  these  are  lacking  it  is 
manifest  that  the  systematic  relations  are  still  too  incom- 
pletely known  to  us,  or  that  the  given  forms  have  died  out. 
Every  species  can  therefore  be  compared  with  its  own 
ancestors  or  with  other  descendants  of  the  same  ancestors. 

This  consideration  leads  us  to  the  recognition  of  two 
different  types  of  relationship,  and  therewith  also  of  two 
groups  of  crossings  between  allied  species,  which  have  to 
be  kept  absolutely  apart.  One  of  them  we  shall  call  the 
avunculary,  the  other  the  collateral.  In  the  first  case  we 
cross  a  form  with  an  "avunculus"  or  ancestor  in  the  direct 
line,  in  the  latter  case  with  one  of  its  lateral  relatives. 
Obviously  the  first  relation  is  very  simple  while  the  latter 
is  more  complicated. 

Every  character  and  every  unit  corresponding  to  it, 
which  in  a  crossing  is  present  in  one  species  and  lacking 
in  the  older  one,  forms  a  special  point  of  difference. 
Hence  the  simplest  case  is  the  one  in  which  there  is  only 
one  such  difference  between  the  two  parents  of  a  cross. 
But  generally  several  of  them  exist. 

Now  in  such  a  cross,  the  differing  factors  evidently 
do  not  find  any  antagonists  in  the  sexual  cells  of  the  other 
parent.  When,  during  fertilization,  the  pronuclei  unite 
into  a  double  nucleus,  all  the  other  units  are  present  in 
pairs.  Not  so  the  differing  ones ;  they  lie  unpaired  in  the 
hybrid. 


258  Fertilisation  and  Hybridisation 

If  we  apply  this  reasoning  to  our  conception  of  the 
arrangement  of  the  units  in  rows  on  the  nuclear  threads, 
the  immediate  result  would  be  that  their  cooperation  must 
be  disturbed.  The  threads  no  longer  fit,  neither  during 
fertilization  and  in  vegetative  life,  nor  later  when  the  units 
are  exchanged  before  the  formation  of  the  sexual  cells. 

If  we  imagine  two  corresponding  chromosomes  of  the 
two  pronuclei  placed  exactly  side  by  side,  and  in  such  a 
way  that  every  unit  of  the  one  has  the  corresponding  unit 
of  the  other  for  a  neighbor,  this  will  occur  in  a  species- 
cross  only  as  far  as  the  point  of  difference.  Here  one  nu- 
clear thread  has  one  unit  more  than  the  other.  The  latter 
has,  so  to  say,  a  gap. 

The  greater  the  number  of  points  of  difference,  the 
more  numerous  are  these  gaps,  and  the  more  will  the  co- 
operation of  the  two  nuclei  be  interferred  with.  And  this 
must  diminish  the  vitality  of  the  germ  or  at  least  the  nor- 
mal development  of  all  characters. 

If  the  differences  between  the  two  parents  are  too  nu- 
merous, a  crossing,  as  is  well  known,  remains  quite  with- 
out effect.  Crossings  between  species  belonging  to  dif- 
ferent genera  succeed  in  very  rare  cases  only,  indeed 
within  by  far  the  most  genera  even  the  ordinary  system- 
atic species  are  not  fertile  when  united.  Genera  such  as 
Nicotiana,  Dianthus,  Salix,  and  others,  which  are  rich  in 
hybrids,  are,  as  a  rule  the  very  ones  in  which  the  species 
are  exceedingly  closely  related  to  each  other. 

Even  if  the  agreement  of  two  species  is  great  enough 
for  mutual  fertilization,  the  life  of  the  hybrid  is  by  no 
means  assured  thereby.  Some  of  them  die  as  seeds  with- 
in the  unripe  fruit,  as  has  been  specially  described  by 
Strasburger  for  the  hybrid  seeds  of  Orchis  Morio  after 
fertilization  with  0.  fusca. 


Sterility  of  Hybrids  259 

Others  become  young  plantlets,  but  are  too  weak  to 
develop  any  further,  and  perish  during  the  first  weeks 
after  germination,  as  I  have  frequently  seen,  for  example 
after  crossings  of  Oenothera  Lamarckiana  and  0.  muri- 
cata.  Or  only  the  most  vigorous  individuals  continue  to 
grow,  while  the  weaker  ones  perish,  and  this,  in  diocious 
plants,  sometimes  results  in  the  male  seedlings  perishing 
while  some  of  the  more  vigorous  female  ones  develop 
flowers,  as  Wichura  observed  in  several  willows.  Finally 
there  might  originate  hybrids  that  grow  vigorously,  but 
do  not  flower  at  all  or  only  incompletely,  or  begin  too  late 
to  do  so.  There  is  a  whole  series  of  cases  between  the 
unsuccessful  crossings  and  the  development  of  hybrids 
into  adult  plants.  And  on  the  whole  this  series  runs 
parallel  with  the  increasing  systematic  relationship. 

If  the  hybrid  has  succeeded  in  reaching  the  period  of 
flowering,  that  is,  the  period  of  the  formation  of  the  sex- 
ual cells,  a  new  difficulty  arises  at  the  moment  of  the 
exchange  of  the  units.  Whereas,  up  to  that  time,  the  co- 
operation of  the  two  pronuclei  was  more  or  less  disturbed, 
now  the  gaps  become  very  important.  Hence  the  quite 
common  phenomenon  that  the  production  of  egg-  and 
sperm-cells  fails  more  or  less  completely,  that  the  hybrids 
either  produce  no  ovules  that  are  capable  of  being  fer- 
tilized, or  no  good  pollen,  or  neither.  They  are  more  or 
less  or  even  completely  sterile.  They  either  form  no  seed 
at  all,  or  only  an  insufficient  quantity.  Only  where  the 
differences  between  the  parents  are  quite  small,  does  one 
succeed  in  harvesting  any  seed,  and  even  here  frequently 
only  a  little. 

How  the  unpaired  characters  behave  during  the  ex- 
change, when  they  are  not  numerous  enough  to  make  a 
failure  of  the  entire  process,  is  at  present  unknown.  Ex- 


260  Fertilisation  and  Hybridization 

perience  teaches,  however,  that  in  these  cases  the  descen- 
dants of  the  hybrids  do  not  display  that  multifariousness 
of  type,  nor  those  splittings  that  are  characteristic  of 
variety-hybrids.  They  usually  all  resemble  each  other 
and  their  parents,  the  original  hybrids,  and  this  constancy 
persists  through  the  course  of  generations.  Accordingly 
there  originate  races  of  hybrids  which,  apart  from  their 
possibly  diminished  fertility,  can  hardly  be  distin- 
guished from  true  species.  Sometimes  they  are  found 
wild,  as  for  example  a  hybrid  race  between  two  Alpine 
roses  and  other  races  of  the  kind  in  the  genera  Anemone, 
Salvia,  Nymphaea,  etc.  Sometimes  they  have  been  ob- 
tained artificially  or  have  accidentally  originated  in  the 
gardens.  The  genus  Oenothera  is  exceptionally  rich  in 
such  hybrid  races,  especially  in  the  sub-genus  of  the  com- 
mon evening-primroses,  Onagra.  Very  frequently  such 
hybrids  are  simply  described  as  species,  on  the  one  hand 
because  they  can  be  reproduced,  without  deviation,  from 
seeds,  and  on  the  other  hand  because  systematic  works 
frequently  do  not  sufficiently  consider  the  elementary 
species.  The  distinguishing  of  the  latter  from  hybrid 
races  is  frequently  by  no  means  easy. 

The  purpose  of  my  explanations  compels  me  to  restrict 
myself  to  simple  and  clear  cases.  In  nature  these  occur 
relatively  rarely,  and  the  individual  elements  of  the  phe- 
nomena are  usually  commingled  in  most  motley  variety. 
By  far  the  greater  number  of  crossings  take  place  between 
parents  whose  mutual  relations  do  not  wholly  fit  either 
the  one  or  the  other  concept,  but  where  the  characteristics 
of  the  different  types  of  hybrids  are  intermingled.  I 
cannot  consider  these  cases  here;  they  are  of  too  com- 
plicated a  nature  for  an  address. 

Only  one  point  I  wish  to  touch  upon.    In  the  preceding 


Mutation-Periods  261 

pages  I  have  always  taken  for  granted  that  the  species 
and  varieties  are  in  their  ordinary  and  unchanging  state. 
But  this  is  by  no  means  always  the  case.  The  origination 
of  new  species  and  varieties  demands  that  their  immutabil- 
ity should  not  be  absolute,  or  at  least  should  be  suspended 
from  time  to  time.  Experience  confirms  this  by  showing 
that  there  are  periods  in  the  life  of  species,  during  which 
they  are,  so  to  speak,  especially  inclined  to  produce  new 
types.  At  that  time  they  produce  the  new  varieties  and 
species,  not  only  once  but  repeatedly,  and  not  only  a  single 
one,  but  frequently  a  considerable  number.  Genera  rich 
in  species,  such  as  the  pansies  and  the  rock-roses,7  are  the 
remains  of  such  periods  of  variability,  and  everywhere  in 
nature  we  meet  with  similar  ones.  In  garden-plants  we 
see,  from  time  to  time,  periods  during  which  certain 
varieties  occur  by  preference,  as  the  double  dahlia  of 
about  the  middle  of  the  last  century,  the  forms  of  toma- 
toes in  recent  decades,  and  numerous  other  instances 
teach  us.  On  its  first  appearance  the  gardeners  call  the 
new  form  a  conquest,  the  later  appearances  are  only  repe- 
titions, and  are  therefore  of  only  very  secondary  practical 
value. 

The  power  of  reproducing  one  or  more  new  species 
indicates  a  condition  of  unstable  equilibrium  of  the  given 
internal  units.  In  the  nuclei  the  new  characteristic  is  al- 
ready invisibly  present,  but  inactive.  Certain  causes,  un- 
known to  us,  can  transform  this  into  a  permanent  condi- 
tion. This  state  of  unstable  equilibrium  may  be  main- 
tained in  the  great  majority  of  individuals,  through  a 
series  of  generations,  as  is  the  case  with  my  Oenotheras. 
But  from  time  to  time,  sometimes  in  individual  cases 
every  year,  there  is  a  shock,  and  the  equilibrium  becomes 

7Sonnenrdschen  (Helianthemum).     Tr. 


262  Fertilisation  and  Hybridisation 

stable.     The    given    individuals    overstep   their   bounds, 
abandon  the  earlier  type,  and  form  a  new  species. 

It  is  evident  that  in  crossings  such  unstable  units  will 
behave  differently  from  normal,  stable  ones.  Their 
chance  of  becoming  stable  is  evidently  considerable,  ow- 
ing to  the  phenomena  of  fertilization  and  the  exchange  of 
units.  In  this  way  constant  races  originate,  at  least  in  the 
genus  Ocnothera,  and  this,  on  the  one  hand,  with  the  re- 
spective characteristic  in  an  unstable  condition,  or  in  other 
words,  in  a  state  of  mutability ;  and  on  the  other  hand  with 
stable  equilibrium  corresponding  to  a  new  species.  But 
researches  in  this  field  are  only  in  their  beginning,  and  do 
not  yet  permit  of  a  detailed  analysis.  Besides  they  repre- 
sent, for  the  present,  a  case  in  themselves. 


In  conclusion,  on  reviewing  the  course  of  our  deduc- 
tions, we  see  that  hybrids  follow  normal  fertilization  quite 
closely,  the  more  completely  the  less  numerous  and  the  less 
pronounced  the  points  of  difference  between  the  parents 
of  the  crossing.  If  these  are  of  such  a  kind  that  the  num- 
ber of  units  in  one  parent  is  different  from  that  in  the 
other,  disturbances  take  place  which,  if  of  lesser  influence, 
diminish  the  fertility  of  the  hybrids,  and  if  of  greater  sig- 
nificance, affect  their  own  power  of  development,  or  even 
make  the  crossing  a  failure.  If  these  units  are  present 
in  equal  numbers  on  both  sides,  and  if  the  differences  are 
limited  to  latency  in  one  parent  and  activity  in  the  other, 
the  normal  process  is  not  at  all  disturbed,  but  striking 
phenomena  occur,  which  find  their  explanation  in  the  pe- 
culiar manner  in  which  the  parental  inheritances  co-oper- 
ate in  the  hybrid  and  in  the  formation  of  its  sexual  cells. 

This  co-operation  is  reflected  in  the  life  of  the  nuclei. 


Conclusion  263 

In  fertilization  the  nuclei  of  father  and  mother  simply 
touch  each  other.  In  the  course  of  development  the  con- 
tact becomes  gradually  closer,  bringing  their  equivalent 
elements  as  near  to  each  other  as  possible,  in  such  a  way 
that  the  latter  finally  all  lie  side  by  side  in  pairs.  But  the 
pronuclei  by  no  means  lose  their  independence  thereby, 
and  for  the  purpose  of  every  nuclear  division  they  sepa- 
rate their  component  parts  more  or  less  distinctly.  Shortly 
before  their  separation,  their  leave-taking,  they  are  still 
the  same  as  before.  But  now  they  exchange  their  indi- 
vidual units,  and  thus  cause  the  creation  of  those  countless 
combinations  of  characters,  of  which  nature  is  in  need  in 
order  to  make  species  as  plastic  as  possible,  and  to  em- 
power them  to  adapt  themselves  in  the  highest  degree  to 
their  ever  changing  environment. 

This  increase  of  variability  and  of  the  power  of  indi- 
vidual adaptation  is  the  essential  purpose  of  sexual  repro- 
duction. It  can  be  attained  only  by  a  mutual  combination 
in  all  conceivable  forms  of  the  peculiarities  developed  in 
different  individuals  in  different  directions  and  degrees. 
To  this  end  the  pronuclei  mutually  exchange  their  units 
from  time  to  time,  and  by  assuming,  on  the  ground  of  ex- 
periments with  hybrids,  that  this  takes  place,  on  the  whole, 
according  to  the  laws  of  chance,  that  is,  according  to  the 
theory  of  probability,  we  have  gained  a  basis  which  al- 
lows us  to  probe  to  its  very  bottom  this  most  significant 
and  mysterious  process. 


INDEX 


Acetabularia,  164. 

Acids,  tannic,  12,  15. 

Actinophrys  Sol,  157. 

Adaptations,  parallel,  13. 

Aggregation,  153. 

Aleurone  grains,  131,  155 

Algae,  102,  135,  148,  149. 

Alkaloids,  12. 

Allium  Cepa,  185. 

Alternation    of    generations,    19, 
236. 

Amyloplasts,  130,  146. 

Ancestral  plasms,   53. 

Anemone,  260. 

Ant-plants,  14,  156. 

Aphids,  32. 

Apical  cell,  84. 

Archiplasts,  67. 

Artemia,  229. 

Ascaris,  227;  lumbricoidcs,  178; 
megalocephala,  145,  177,  178. 

Asclepiadaceae,  14. 

Ascomycetae,  102. 

Ascospores,  165. 

Ascus,  102. 

Atavism,  16,  23,  25;  specific,  60. 

Atoms,  13;  memory  in,  48;  will- 
power in,  48. 

Aucuba,  106. 

Avunculus,  257. 

Axolotol,  229. 

Batrachoseps,  231. 

Bees,  32. 

Begonia,  106,  146. 
phyllomaniaca,  199. 

Begonias,  29,  99,  105,  205. 

BELLONCI,  229. 


BEYERINCK,   16,  98,  99,   118,  119, 

120. 
BOVERI,  218,   230,  232,  233,  235, 

254. 

Brachystola  magna,  233. 
Bras  sic  a  oleracea,  182. 
BRAUER,  229. 
BREFELD,  96. 
BROWN- SEQUARD,  65. 
BRUCKE,  126,  183 
Bryophyllum  calycinum,  98. 
Bryopsis,  143,  176. 
Buds,    adventitious,    98;    callus, 

97,  98 ;  root,  98. 
Bud-formation,  51,  97. 
Bud-variation,    16,   24. 
Cactacese,  14. 
Calcium  oxalate,  15. 
Callus,  97. 
Callus-buds,  97,  98. 
Cambium,  97. 
Carbon,  38. 

Cardamine  pratensis,  98. 
CARRIERS,  29. 
CASPARY,  106. 
Catasetum  tridentatum,  18. 
Cecidium,  118. 
Cecidomia  Poae,  118,  119. 
Cecropia  adenopus,  56. 
Cell-division,     neogenetic,      128 ; 

panmeristic,  128 ;  phyletic,  107 ; 

somatarchic,  107;  somatic,  107. 
Cell-pedigrees,  80. 
Cell-plate,  161. 
Cell-ring,  162. 
Cellulose,  secretion  of,  47. 
Chara,  144,  163. 


266 


Index 


Characeae,  148,  159. 

Characters,  composite  nature  of 
specific,  11 ;  hereditary,  11 ;  mu- 
tual independence  of,  11 ; 
transmission  of  hereditary, 
179. 

Chelidonium,  106. 

Chlorophyceae,  145. 

Chlorophyll,  15. 

Chlorophyll-bodies,  origin  of, 
129. 

CHMIELEVSKY,  172. 

Chromatin,  secretion  of  from  nu- 
clei, 242. 

Chromoplasts,  147. 

Chromosomes,  177,  178. 

Circaea,  16. 

Cladophora,  132,  148. 

Clarkia,  106. 

Cleistogamy,  32. 

CAMPBELL,  175. 

CANNON,  238. 

Coccodules,  45. 

C  odium,  143. 

Coeloblasts,  187. 

Compounds,  chemical,  12. 

Conferva  glomerata,  134,  135. 

CONKLIN,  218,  229,  242. 

Copper-beech,  21.  v^ 

Cormophytes,  83. 

Corn,  211. 

Correns,  211. 

Crabs,  fresh-water,  238. 

CRAMER,  85. 

Crassulaceae,  98,  106. 

Crepidula,  229,  242. 

Crinoid,  200. 

Cross-fertilization,  29. 

Crown-graft,  211. 

CRUDER,  159. 

Crystalloids,  131. 

Cucumis,  106. 

Cycadacese,  226. 


Cyclops,  229;  vulgaris,  228. 

Cynipidese,  19,  119. 

Cytissus,  224;  Adami,  223;  La- 
burnum, 223 ;  purpureus,  223. 

Cytoplasm,  202;  composed  of 
pangens,  200;  defined,  195. 

Dahlia,  double,  261. 

Daphnoidae,  94. 

DARWIN,  C.,  3,  14,  22,  23,  24,  29, 
30,46,50,51,58,59,62,63,64, 
71,  73,  91,  99,  109,  153,  199,  207, 
212,  214,  215. 

DARWIN,  FRANCIS,  14. 

Datura  Stramonium,  224. 

DEBARY,  165,  171,  172. 

DELAGE,  \\. 

DELPINO,  26. 

Derbesia,    143. 

Dianthus,  258. 

Diatomes,  149. 

Dichogeny,  15,  16,  24. 

Digitalis  lutea,  181 ;  purpurea, 
181. 

Dimorphism,  27. 

Dioecism,  27. 

DIPPEL,  159,  160. 

Dipsacus  sylvestris,  20,  213. 

Diptera,  93,  94,  101,  119. 

Dispermia,  254. 

Drosera,  14,  153;  intermedia,  153; 
.  rotundifolia,  153. 

Duality,  principle  of,  220,  221. 

Echinidae,  169. 

EIMER,  210. 

EISEN,  218,  231. 

Elaioplasts,  149. 

ELSBERG,  L.,  44,  45,  46,  48. 

Embryo-sacs,  164. 

ENGELMANN,  234. 

Epithemia,   173. 

Equisetum,  83,  100 ;  palustre,  83 ; 
arvense,  86,  87. 

ERRERA,  218,  231. 


Index 


267 


Euglenae,   149,    156. 
Euglenidse,  133. 
Euphorbiaceae,  14. 
Evening-primrose,   222. 
Eye-spot,  149. 

Ferns,  IS;  prothallia  of,  108. 
Fertilization,   169,  170,  171,  180; 

essence    of,    31,    32,    170,    194, 

226;  263;  in  cryptogams,  173; 

in    phanerogams,    176;    result 

of,  253. 
Flax,  18. 
FLEMMING,  125,  163,  169,  183, 

230. 

Florideae,  208. 
FOCKE,  181,  182,  211. 
FOL,  169,  229. 
Fuchsias,  26. 
Fucus,  175. 
Fungi,  96. 
Gall-roots,  120. 
Galls,  118;  cynipid-,  118. 
GARTNER,  28. 
Gemmule,  4,  64,  71,  206. 
Geum  album,  182;  urbanmn,  182. 
Germ-plasm,  90,   110,   121. 
Germ-tracks,    55,    89,    103;    pri- 
mary, 93,   104;   secondary,  95, 

105. 

GODLEWSKI,  70,  200. 
GOEBEL,  16,  84,  85. 
GOETHE,  219,  221,  228,  229. 
GOTTE,  82. 

Graft-hybrids,  65,  210. 
Granules,  4. 
Grasses,  15. 
GRUBER,  187,  199,  201. 
HABERLANDT,  142,   185,   186,  203, 

204,  226. 

HACKER,  218,  228. 
HAECKEL,  E.,  38,  39,  41,  44,  45, 

46,  47,  48,  169,  183,  184,  194, 

225. 
Halosphaera,  148. 


HANSGIRG,  145. 

HANSTEIN,  66,  114,  126,  140,  185, 
205. 

Helianthemum,  261. 

Hereditary  characters,  24. 

Hereditary  factors,  independent, 
11,  34;  miscible,  24,  34. 

HERTWIG,  O.,  169,  183,  195,  225. 

HERTWIG,  R.,  169. 

Heteroplastids,  82. 

Heterostyly,  18. 

HOFF,   VAN'T,  38. 

HOFMEISTER,  128,  129,  131,  134, 
206,  230. 

Homoplastids,  82. 

HOOKER,  J.  D.,  v. 

Hordeum  trifurcatum,  106. 

Horse-tails,  prothallia  of,  108. 

Horse,  zebra-like  stripes  of,  23. 

Hoya  trifurcatum,  106;  car- 
no sa,  106. 

Hyaloplasm,  150. 

Hybrids,  221;  disjunction  of 
characters  of,  28;  progeny  of, 
254;  species, — 251;  variety, — 
251,  254;  vegetative  splittings 
of,  240. 

Hybridization,  27. 

Hydrodictyon,    164. 

Hydroids,  90. 

Idioplasm,  57. 

IKENO,  227. 

Insects,  238. 

Isogametes,  176. 

JAGER,  89. 

JOHANNSEN,  247. 

JOHOW,  144. 

JULIN,  145. 

KELLOGG,  V.  L.,  vi. 

KLEBS,  126,  133,  135,  140,  141,  149, 
151,  157,  188,  199,  201. 

KOLDERUP-ROSENVINGE,    208. 
K6LLIKER,   229. 
K6LREUTER,   28. 


268 


Index 


KORSCHELT,  185,  186. 

KRABBE,  151 

Latex-vessels,  208. 

Liegesbeckia,  106. 

LEMOINE,  222. 

Levisticum,  106. 

Life-processes,  two  kinds  of,  39. 

Liliaceae,  178. 

Linaria,  genistae folia,  181 ;  pur- 
purea,  181 ;  vulgaris,  181. 

LINDEMUTH,  211. 

Liverworts,  96. 

Lycopersicum,  106. 

Lysimachia  vulgcuris,  26. 

MAC  FARLANE,  182,  225. 

Maize,  211. 

Marchantia  polymorpha,  96. 

Medicago,  falcata,  182;  sativa, 
182. 

Membranes,  autonomy  of  limit- 
ing, 160;  limiting,  157;  plas- 
matic,  157,  158. 

MENDEL'S  law,  253. 

Mentha,  16. 

Metamorphosis,  73. 

MEYER,  A.,  130,  145,  149. 

Micrococcus,  231. 

Microsomes,  150. 

Mikroplasts,  67. 

Mohl,  81,  126,  131,  132,  134,  160, 
205. 

Molecules,  13;  chemical,  37;  liv- 
ing, 49. 

Molluscs,  238. 

Monachanthus,  18. 

Monoecious  plants,   17,  24. 

Monoecism,    27. 

Monotropa,  98. 

Mosses,  96. 

MOTTIER,  viii. 

MULLER,  114. 

Muller's  bodies,  156. 

Muscineae,  96,  104 

Mutability,  214. 


Mutation-periods,   261. 

Myanthus,   18. 

Mysostoma,  229. 

NAGELI,  viii,  24,  57,  58,  59,  81. 

Nasturtium  officinale,  98. 

NAUDIN,  181,  225. 

Nectarines,  17. 

Nematus  capreae,  118;  viminalis, 
119. 

Nepenthes,  14. 

Nicotiana,  258. 

Nucleo-molecules,  45. 

Nucleus,  194,  202;  composed  of 
pangens,  200,  215;  double  na- 
ture of,  227 ;  influence  in  cell, 
183;  origin  of,  198. 

NUSSBAUM,  100,  188,  199,  201. 

Nymphaea,  260. 

Oedogonium,  188. 

Oenothera,  260,  261,  262;  La- 
mar  ckiana,  259;  muricata,  259. 

Oil,  formation  of,  149;  etherial, 
12,  15. 

Onagra,  260. 

Orchidaceae,   14,   178. 

Orchis  fusca,  258;  Mario,  258. 

Organism,  elementary,   126. 

Oscillariae,  82. 

OVERTON,    171. 

Pangenesis,  63,  73;  intracellular, 
defined,  215. 

Pangenosomes,  viii. 

Pangens,  viii,  7,  49,  70,  74,  193, 
195,  215 ;  active  and  latent,  197, 
199,  254;  transportation  of, 
201,  202,  204,  215;  multiplica- 
tion of,  212,  213. 

Pansies,  216. 

Papaver  hybridum  L.,  28 ;  somni- 
ferum  polycephalum,  20. 

Peperomia,  106,  146. 

Peregenesis,  44. 

Peronosporales,  165. 

Petalody  of  bracts,  73. 


Index 


269 


Petals,  increase  of,  20. 

PFEFFER,  149,  157. 

PFLUGER,  41. 

Phaseolus  multiflorus,  180,  181 ; 
vulgaris  nanus,  180. 

Physiological  units,  51. 

Plasma-membrane,  42. 

Plasson,  45. 

Plastidules,  44,  46. 

PLATNER,  163. 

Poa  nemoralis,  119. 

Polyps,   colony- forming,  94. 

Polysiphonia,  208. 

Potato,  15. 

Primula  acaulis  var.  caulescent, 
23,  60. 

Primulaceae,  18. 

Principles  of  Biology,  51. 

PRINGSHEIM,  96,  104,  164,  186, 
187,  189. 

Pronuclei,  228. 

Propagation,  asexual,  247. 

Protein  and  protoplasm,  41 ;  ar- 
tificial synthesis  of,  43;  living, 
41. 

Prothallium,  236. 

Protomyces  macrosporus,  164. 

Protoplasm,  41,  125,  126;  arti- 
ficial synthesis  of,  43 ;  com- 
posed of  pangens,  37,  43,  195, 
197,  216;  currents  in,  205,  216. 

Protoplasts,  125;  connection  of, 
208;  regeneration  of,  139. 

Pseudosomatic  tracks,  100. 

Pyrenoids,  149. 

Races,  how  improved,  31,  32. 

Reduction  of  chromosomes,  237. 

Reduction-division,  240. 

REES,  85. 

REGEL,  98,  99. 

Regeneration,  95,  139,  143. 

Rejuvenation,  99. 

Relationship,  systematic,  73. 


Reproduction,      significance     of 

sexual,  247,  248,  263. 
Rheum,  106. 
RIMPARA,  22,  222. 
ROBINSON,  Miss,  14. 
Rock-roses,  261. 
Raphanus  sativus,  182. 
Roses,  Alpine,   260. 
Roux,  178,  201,  230. 
RUCKERT,  228. 
.  Rumex  Acetosella,  16,  98. 
Russow,  209. 
SACHS,  70,  81,  85,  99,  115,  129, 

134,  143,  147,  150,  151,  174. 
Sagitta,  94. 

SAGIURA,  SHIGETAKE,  39. 
Salamander,  231. 
Salix,  258;  purpurea,  119. 
Salvia,  260. 
Saprolegniaceae,  164. 
Sarracenia  purpurea,  14. 
SCHACHT,  174. 

SCHIMPER,  14,  130,  145,  156,  186. 
SCHLEIDEN,  79. 
SCHMIDT,  143. 
SCHWANN,  79,  114. 
SCHMIDTZ,  102,  129,  145,  148,  150, 

173,  175,  176,  187. 

SCHWENDENER,    143. 

Scytosiphon  lomentarium,   176. 
Sea-urchins,  200,  235. 
SELENKA,  169. 
Self-fertilization,  29,  30. 
Sempervirum  tectorum,  20. 
Sexual  characters,  secondary,  18. 
SHULL,  256. 
SINETY,  238. 
Siphoneae,  143. 
Siphonocladiaceae,  145,  187. 
Slredon,  229. 

Somatic  tracks,  89,  100,  103,  105. 
Species,  how  originate,  256. 


270 


Index 


Species-hybrids,   249. 

Specific  characters,  composition 
of,  34. 

SPENCER,  SO,  51,  et  seq.,  58,  59,  60. 

Spermatozoids,  origin  of,  174. 

Spitogyra,  132,  139,  149,  169,  171, 
173,  187,  226,  227;  Weberi,  171, 
172;  Zygospore  of,  171. 

STAHL,  147. 

Star-fish,  235. 

STRASBURGER,  viii,  99,  115,  125, 
129,  131,  135,  137,  150,  159,  160, 
161,  162,  170,  174,  177,  178,  183, 
186,  187,  202,  223,  225,  230,  236, 
238,  242,  258. 

SUTTON,  218,  230. 

Swarm-spores,  149,  164. 

Sword-lilies,  222. 

Symplasts,  209. 

Synapsis,  240,  253. 

TANGL,  185,  209. 

Tape-worm,  213. 

Tax  odium,  100. 

Thallophyta,  95,  104,  183. 

Thistles,  98. 

Tonoplast,  152. 

Toxopneustes,  229. 

Transportation-hypothesis,  207. 

Trifolium  hybbridum  L.,  28. 

Trimorphism,  27. 

Trophoplast,  42,   130,   144. 

Turgidity,  cause  of,  150. 

TURPIN,  114. 

Ulothrix,  164. 

Uridineae,  19. 

Urtica,  106. 

Vacuoles,  150;  contractile,  156; 
pulsating,  156;  wall  of,  152. 

Valonia,  145. 

VAN  BENEDEN,  145,  177,  218,  227, 
228,  229. 

Vaucheria,  140,  141,  142,  160,  187. 

Vanilla  planifolia,  149. 


Variability,  correlative,  73;  fac- 
tors of,  74;  fluctuating,  214; 
phylogenetic,  74;  species-form- 
ing, 214;  two  kinds  of,  214. 

Variations,  sudden  origin  of,  22. 

Variety-hybrids,  249,  254. 

Varieties,  how  fixed,  31,  32;  in- 
constant, 251;  result  of  cross- 
ing, 249. 

VELTEN,  205,  206. 

Verbascum  blattaria,  182;  phoe- 
niceum,  182. 

VERLOT,  29. 

'Veronica  longifolia,  224. 

Vertebrates,  93. 

VILMORIN,  25,  91. 

VINES,  14. 

VOCHTING,  96,  104,  114,  116. 

VOLKENS,  G.,  14. 

WAKKER,  98,  99,  131,  149,  155. 

WEBBER,  211,  227. 

Weigelias,  26. 

WEISMANN,  50,  53,  et  seq.,  58,  59, 
60,  65,  68,  79,  80,  90,  91,  103, 
110,  202,  210. 

WEISS,  147. 

WENT,  131,  137,  140,  154,  155,  156, 
161,  206. 

Wheat,  222. 

Wheat-hybrid,  222. 

WHEELER,  229. 

Whorls,  25. 

WICHURA,  32,  259. 

Willows,  33,  259. 

WILSON,  218. 

Worms,  238. 

Xenia,  210,  211. 

Yucca,  16. 

ZACHARIAS,  42,  137,  163,  175. 

Zea  Mays,  211. 

Zimmermann,  144. 

Zygnema,  173,  188. 

Zygosporeae,  173. 


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